My mother-in-law, now in her seventies, was in a similar position as the daughter in OP's post.

When she was seven her mother died in her thirties from breast cancer. Her father then married a 20-year-old. They went on to have two more girls. My mother-in-law was clearly resented by her father's new wife, and you can still see today how she treats my mother-in-law differently. She treats her like a second-class citizen, and my wife found out that when she was young, her grandmother would give her less pocket money and cheaper gifts etc than her cousins, who are the children of her biological children.

This is an awful thing to repeat, but my mother-in-law recently went to see her step-mother on her birthday and when she arrived, one of her step-mother's friends said 'I believe you're a friend of the family' to my mother-in-law.

It's clear she resented the existence of my mother-in-law from the very start and it has messed her up for a lifetime. She tries to treat my wife like a parent, which puts a strain on our relationship. She sometimes acts like a lost, unwanted child, which is kinda understandable. Her relationship with my wife looks, to me, not like a mother-and-daughter relationship, but one where my wife is someone who's always available, almost like a concierge and a counsellor.

If I was the OP I would seriously consider my relationship, because I've witnessed your potential future and it's filled with bitterness and sadness, and decades of therapy that appears to have achieved very little.