Colleen: When you grow up with someone and you’ve always known that they’re outstanding, it’s easy to take it for granted. When people talk to me about Valerie, they’ll tell me about things she’s accomplished, things that she hasn’t told us about at home, and I’m like: “Really? Wow!” She’s amazing, and we consider it normal.

Valerie has never come home and boasted about what she’s done, even when we were children. But I’ve always known she would be successful. She has always been very eloquent. As a child, she read constantly. And I heard the way the teachers spoke about her and watched how other students reacted to her. When she became deputy head girl at our school, it was a phenomenal achievement for a black girl back then.

Our parents brought us here from Guyana when I was six and Valerie was nine. Their lives were built around making sure we could achieve what we were capable of and that we had a really good education; that was their main reason for bringing us here. They were fairly strict. We knew what we had to do: wash up, do homework, that kind of thing. If you overstepped the rules, you’d be sent to your room, have to do the washing-up alone, get a smack. But I think those boundaries helped us feel safe, and the house was warm and loving and fun. Our friends would come round, and our family parties were renowned. We’d dance and sing and eat.

As we were growing up, Val took on the role of caring for us when our mother wasn’t there. Both our parents worked, so she would look after me and our younger brother when we got home from school and do things around the house, and she still managed to excel at school. She kept me on the straight and narrow as a teenager. She would take me to the library and make sure I was doing my homework. She would read a book and tell me: “You need to read that.” She was a real rock. But she would also take me to places I’d never have been able to go to on my own.

Valerie has always looked after me, and I always go to her if I’m trying to work something out. She’s good at listening, and her real skill is that she doesn’t give advice, she helps me come to my own conclusion. Even if I’ve made a decision she isn’t in favour of, she won’t tell me not to do it, and if it goes wrong she’ll never say: “I told you so.” What she’ll say is: “Okay, how are we going to get you out of this one?”

We were brought up to be good at debating and arguing — so although we do argue, it never cuts across the fundamental strength of our relationship. Other families seem to have a row, and then not be on speaking terms for ages. But with us, you say what you want to say, and you might disagree about it, but you respect the other person’s point of view, and you move on.

Most people don’t see her vulnerable side, which I do. I have sometimes worried about how hard she works and how tired she gets, although I have absolute faith that she’ll be fine. But I know how to react when I can see that she needs a bit of comfort, or to unwind a bit. If I know she’s coming home from a work trip, or going through a busy time, I’ll go and visit her, or ask her to come over and have some dinner. Or I’ll tell her we’re having a dinner party and she’s cooking — that’s her way to relax.

When we’re together, we keep politics out of it — I try to stay separate from that part of her life. I’m her sister, and I’m here to support her in her work, but not necessarily to be involved with it. I’ve never felt competitive with Val — we’ve always had different dreams, and our personalities and skills complement each other. It’s about sharing our strengths and going forward together.

Valerie: Colleen and I have always been close, and that has partly come from sharing the experience of being migrants. We were the first black family in our neighbourhood in suburban Kent, and the first and second black pupils at our school. People who hadn’t seen black people before thought we were a bit strange, and there were occasional incidents of people making racist comments. But we supported each other, and I don’t remember it as being particularly difficult.

I was a very studious child, so Colleen probably played with our younger brother, Michael, more than with me when we were small. But we’ve always had a good relationship. Because I was the older sister, we would do things together, and I would take her to shows when we were teenagers. We saw Diana Ross and Bob Marley together. It never felt like she was tagging along. I just genuinely liked having her around. Although she wasn’t quite a peer — we stayed in the roles of older and younger sister, and she was often very quiet — my friends became her friends too.

We’d visit each other while we were at university, and we still have quite a lot of friends in common, as well as having our own separate friends. We still do a lot of things together: go to the movies, go to the theatre. If I have work-related events that link in with her interests, or sometimes even if they don’t, I’ll ask her if she wants to come with me.

It must have been extraordinarily difficult for my parents to leave Guyana behind when they brought us to England. Their decision to bring us here really came down to education — they were certain that they wanted us to go to university. They left everything they knew, their careers, all their friends. But I only became aware of it as a huge sacrifice much later on, when I was in my teens or twenties, and becoming active in issues around equality.

Ours was a household where books were read and ideas were debated. We saw our parents with their friends, talking about what was happening in the world. If you argued your point well, the family would acknowledge that.Excellence, and striving for excellence, was taken for granted. If my parents thought that we weren’t doing as well as we could at something, they would ask us what went wrong. There was a feeling that if you’d managed to get 98%, well, how come you hadn’t got 100%? If you weren’t great at something, they would support you to do it as well as you could. But where they knew we could do better and we’d just been a bit lazy, they’d pick us up on it. We were all brought up to have a lot of confidence. I think learning to stand up for yourself and present your case in a debate helped, and we were encouraged to feel that if we wanted to do something, we could.

Now Colleen and I live five minutes away from each other, and even though we don’t always have time to see each other, we speak on the phone every day. I’m very private, so I don’t necessarily talk a lot about issues that are bothering me. But I know that Colleen is there if I do want to talk to her. And I think the same applies the other way round. I can tell if she’s not all right, and I’ll make sure she’s okay. We’re not really the type to phone up and complain all the time — but it’s good to know that someone is there if you do need to.

I’m not married and I don’t have children. I’ve never regretted that — it’s never been an issue for me. Colleen’s children have always been like my children. When her son was four, he went to school and they asked him to tell a story about his mummy, and he said that he had three mummies. They said: “You can’t possibly have three mummies.” He got very upset and came home and said to Colleen: “I do have three mummies, don’t I? I’ve got you, and I’ve got Grandma, and I’ve got Auntie Valerie!” We had to ring the school and explain that it was actually okay, there was nothing odd going on. But it was true then and it’s still true now that he could go to his granny or come to me, and he had a close relationship with each of us. Both her children text me, ring me or visit me — sometimes just to check I’m not too busy. They do it of their own volition. Colleen and I aren’t just sisters, we are best friends.

I love her, but I also like her.

Times recommends

• Live chat: Thrifty ways to update your winter wardrobe
• Is anybody out there really listening?
• Nutrition Q&A: Will eating late make me fat?