AIBU to think my stepdaughter’s mum is stepping away from her responsibilities?

[islifemorethanmum]

My partner’s 15-year-old daughter moved in with us during the week after spending around two years out of mainstream education while living with her mum.
She now attends school locally and lives with us Monday to Friday, returning to her mum at weekends and during school holidays. We also have a young child together.

Recently, a dental appointment revealed longstanding issues with her teeth that hadn’t been addressed, which has raised concerns about the care she’d previously received.

We’ve now been told her mum is moving from a two-bedroom house into a more expensive one-bedroom apartment, despite having a teenage daughter who still stays with her. To add to this, her mum now wants her to spend even more time with us.

We’re already providing the majority of her day-to-day care, schooling, meals and general support. Her mum still receives child benefit, and maintenance payments haven’t always been consistent.
I completely understand that circumstances change, but I can’t help feeling that her mum is gradually stepping back from her parental responsibilities while we’re expected to pick up more and more.
This isn’t about not wanting my stepdaughter here. It’s about questioning whether it’s reasonable for one parent to keep reducing their involvement.

As a mother I cannot fathom not doing anything and everything I possibly can for my child. I would go through fire and back if it meant supporting and protecting them.

my Dads dad had her overnight every Wed and Fri night and alternating Saturday nights when we first split up (he was offered 50:50). It gradually got less and less until early this year (8 years on) when he just stopped calling. DD is nearly 16 and has a variety of MH issues and this year has been a nightmare (she’s lovely just poorly). He doesn’t even know. I don’t think she and I were surprised and you can’t force him to care so we left it whilst she was in crisis and when she emerged out the other side 4 months later and he’d never called once she elected not to pursue him. It’s not abnormal for bad parents not to want to bother when it’s a bit difficult.

@SheilaFentiman If it is something like that - and assuming mum is working to try ro make ends meet - she shouldn't be seen as not being a good mum to her daughter because she can't afford that extra bedroom.
My husband and I are long-term private renters ourselves, despite both working, so I know how expensive paying a private rent can be only too well.

Also, re the dentist, could money be an issue as to why she hasn't been?
There are absolutely no dentists in our area taking on NHS; we have to pay privately for our 6 yo to have check-ups and any work.