DEAR BEL ,

MY SISTER and I recently found out that we have another sister. We knew Dad had an affair back in the day (with a girl six months younger than me!) but we didn’t know there was a child.

Apparently, he carried on the affair for 16-17 years after we found out, and saw his daughter most weeks until she cut him out of her life several years ago because he wouldn’t introduce her to us. She got in touch with my sister via social media.

We’re both having great difficulty forgiving Dad for his betrayal as he swore on my sister’s life that it had finished. But our wonderful, beloved and adored father was leading a double life right under our noses.

He passed away 18 months ago, and our mum died in March last year.

We are quite convinced she knew nothing about it as she had taken an overdose when she first found out about the affair back in the 1970s. We don’t think she could have lived with the thought of him having a child elsewhere.

We have met our sister, who’s a nice girl — all this is not her fault.

That, too, though is difficult. We don’t want to turn our backs on her because she’s had a lifetime of being denied and it would be cruel to do the same.

But I’m not sure how we can build a relationship around something that causes us pain. She has a family of her own and is quite fulfilled so she won’t be hanging around waiting for us to include her in ours. In fact, we’re not quite sure where she wants it to go either.

I just wish I could contemplate my dad’s photos without calling him awful names and looking the other way.

He was the best dad we could have asked for but now it all seems such a sham — and that feeling never goes away.

I just want to stop thinking about it and get on with my life — I was 65 on Christmas Day and feel it is too late in life for this kind of upset. How do I get it out of my mind? LUCY