I have 2 DSs and have had DD (DSD) since she was about 10. Both her parents dipped out and she got to be mine, her Dad left her with me. I have never formally had custody of her and her mother has always claimed the CB for her. But DD was always scared of her Mum so I left it.
I always told her to call me FirstName and one of my proudest moments was when meeting a counsellor from her school who called me Mrs Blah and I said no I am Mrs Blog - I am DD's SM not her Mum and she said - "oh she says you are her Mum".
I chatted to DD about it later and she said "well, whatever, you are my Mum innit".
I think I am blessed to have got a bonus kid.