I can't comment on the happy relationship part as ime scapegoat children are generally emotionally quite screwed up and need a LOT of work to not fall back into old patterns - something I haven't achieved yet.
When I told my mother I was getting married she looked like thunder, only perked up once she was able to talk about my golden child sister again (who had finally managed to learn to cook at the grand old age of 33).
I am successful in my job and everyday life other than social interactions. Every success I had, she managed to sour somehow.
Be it getting great A-levels (she criticised me for the one A despite me being top of my entire year, getting more A-levels and more at A* than anyone else).
She refused to speak to me for 4 whole weeks while I made plans to move abroad, getting accepted at any university I applied to.
She was so happy when me getting pregnant and then left meant my degree was at risk, but miserable when I did it anyway, despite doing the whole thing as a lone parent.
She was even more miserable when I bought my first house, knowing my sister would never have the money (no doubt by now she has been gifted enough for her own deposit; I had to work for every penny).
She loved it when I got divorced and has conspired with my ex ever since. She has alienated me from the entire family after my last grandparent died.
I severed contact completely many years ago now and I am happier for it.