YA 60s or 70s book about an adopted teenager

[Bookhelp]

Hi, can anyone help with a book from the 60s or 70s, about a teenaged girl who discovers that her aunt is her biological mother?

I thought it was by Honor Arundel, but I can’t find a book of her that matches this description. It’s about that vintage and set in the UK.

The girl is upset by the discovery but is helped by her grandmother as their relationship hasn’t changed. She reconciles with her adoptive mother at the end, and the mother reminisces about how close they were when the girl was a baby.

Any help gratefully received :)

Hopeful bump? It was set in a city, perhaps Edinburgh, and the aunt had babysat the adopted girl and her sister when they were babies.

Was the adopted girl called Sue, and she was one of three children and looked different to the others? If that’s right I can’t remember the title though, sorry…but maybe someone else will!

It's ringing some distant bells.
Is it by Joan Lingard?
She was from Edinburgh.

Thanks very much for your suggestions.

I only remember two sisters, not 3 siblings. The adopted girl was good at maths and thought one of her birth parents might be a musician because “maths and music go together”.

I couldn’t see one from Joan Lingard that matches, though I saw she was a friend of Honor Arundel so that’s definitely the right area.

Might it be The High House by Honor Arundel? Set in Edinburgh too, and yes I remember the maths and music thing (although untrue in my own case, I was musical but not at all mathematical)

Hmm not sure if the description fits with The High House. Emma's parents are killed, she's not adopted, her aunt isn't her mother, her brother is musical.