What is it about sisters that causes them to love and hate each other at the same time?

This Sunday is National Sisters’ Day, causing me to ponder the question.

I’m five years older than my sister, and to this day I remain convinced that she is the favoured child.

Was it just my imagination that she didn’t get told off as much as I did as we grew up? Maybe it was, but maybe it wasn’t. My dad recently reprimanded me for arriving late for a visit.

I could understand why he was annoyed, but the visit was combined with another (ironically, involving my sister) and was logistically difficult. As he told me off, all I could think of was the many times my sister had rolled up late, without them batting an eyelid.

In our house, it is the other way around. My youngest daughter harps on about how much we favour her older sister.

“It’s always about Molly. Molly, Molly, Molly,” she will say, as we prise her away from Facebook and hand the laptop to her sister to fill in university forms or book driving lessons. I can understand why it seems that way, but her sister is 18 and, hopefully, about to change her life, so there is a lot to organise.

I’ve tried to explain – and stress that we will do the same for her – but, like me at that age, she doesn’t listen.

As teenage siblings, my daughters argue about many things – who is sitting where on the sofa, who owns certain items of clothing, who owes who money. Yet they are also friends, who sometimes go out shopping and for meals together.

With the age-gap, my sister and I didn’t argue, but she was easily led and the butt of many a joke I played on her.

As we got older we got on better. For a time, we were good friends. She came to stay at my student house in London and we even went on holiday to Italy together.

But since then we have not been particularly close. I thought that would change when she became a mother, but she is still quite distant.

She leads an exciting cosmopolitan life compared to my dull provincial one. It takes a million attempts to get her to reply to texts and it would be easier to get through to Barack Obama by phone.

When I do see her she is fun and vibrant and we have a great time. My daughters adore her.

This Sunday I will remind my daughters to show each other a little appreciation.

I’ll do the same to my sister – if I can pin her down for long enough.