The stepmother-to-be was presumably not in Margaret's life for very long. I can understand that when she was told that Margaret didn't want to see her any more she accepted it, with or without relief.
What I can't understand is the rest of her family. The bare facts appear to be:
Mum and Dad marry. Margaret is born 7 or 8 years later and is the only child of the marriage. Margaret proves to have learning disabilities.
Mum and Dad split up when Margaret is 12 or 13. Dad takes her to live with him and his parents.
Dad is set to marry again, but unexpectedly falls ill and only lives for a few weeks. In the confusion of his death, he makes an arrangement with friends that Margaret can go and live with them. However, at first she goes to live with her mother. This doesn't work out well.
Mother didn't cope, tried and failed to get help from social services, took up offer of respite care from Dad's friends. However, once Margaret was ensconced there the 'friends' clearly decided she was going nowhere.
Now, this is where things take a turn I can't understand. Why did Margaret's mother accept it when told that Margaret no longer wanted to see her? Why didn't she keep pressing for information about her daughter? Why did the rest of the extended family not do the same?
Maybe I'm being naive but I can't imagine having a child as vulnerable as Margaret and just washing my hands of her in this way.