Maybe I'm wrong (hopefully!) but the way "Joan" has no mixed feelings about getting "Clare" back, it is as if she doesn't feel the understanding for "Jessica's" situation that you would surely expect?
While Claire now calls her “Mum”, Joan tells me that Jessica no longer does. But Joan feels only that she has gained a daughter.
I know she says "Jessica" will always be her daughter but honestly if they had previously had a good relationship, would "Jessica" have stopped calling her "Mum"? I don't think so.
In "Claire's" case she is unambiguously delighted with her new and better family because her other family was a bit rubbish. If they had been close and loving, she would be less keen to dive into a new family. It feels as if "Joan" is also a bit keen on the daughter front.