Will give a bit of background so not AIBU by stealth. 
I've been with my DH for 15 years. When I met him DSD was 2.5, he had already split from her mum a year previously. When we started going out DSD lived in her (maternal) grandparents house with her mum. Although her grandparents were the one's who looked after her really, as her mum would go missing for days on end after being on a boozy bender. DH would see DSD at weekends and on his day off during the week, with her staying over every other Saturday.
Back then his relationship with his ex was volatile to say the least. When I came on the scene it seemed to make things worse with his ex.
Anyway the years passed. His ex went on to have 2 more children to different boyfriends. Things calmed down and we all had a reasonable relationship really. ALthough legally DSD was supposed to be living with her mum, she was living with her grandparents around 98% of the time. And then started school down the road from their house.
His ex and I struck up a bit of a friendship about 13 years ago and started to have a few nights out together. In the end I found her far to intense and demanding and scaled down my relationship with her. But we've remained on friendly terms ever since. (With the odd hiccup thrown in).
DSD moved in with us 9 years ago. Her mum got pissed off at this and caused a bit of trouble around that time. Her mum isn't very maternal really, will palm her kids off on anyone and has a bit of a mouth on her.
Fast forward to today. DSD is almost 18. Over the years she's had a volatile, sometimes violent relationship with her mum. Atm they're getting on ok. DSD was beaten up whilst on holiday earlier this week. She had to cut her holiday short and get the first flight home. She has a broken arm and a black eye.
Today her mum came to see her. It was pleasant we all sat chatting, making plans for DSD's 18th whilst having a cuppa. I said to DSD that I'd bought her some bubble bath if she wanted a bath later, and would help her wash her hair. (She has a cast on her arm and can't get it wet). Her mum then said she was going after being here for about an hour.
I then later thought how sad it was for DSD that her mum should be the one buying her bubble bath and washing her hair and she's not I am. I thought (this is the AIBU bit) doesn't this woman have any shame? How can she sit there whilst her daughter is in pain and let someone else be doing all the caring for her. I look at my dds and there's no way I could do that. I'd have to be right there doing all the things they needed doing. It made me sad really, that this woman has missed out on so much in regards to DSD. The good and the bad stuff that comes from being a mum. And DSD has missed out on having that loving relationship with her mum. She has had that type of relationship with me and her nan, but it's no substitute for the real thing.
I probably am BU but just thought I'd vent.