Any step mums ?

SORRY LONG POST !!!

Any step mums here that can offer any advice?

Back story : previous marriage ended in 2016, I have 4 children all over 20 years old, I also have 4 grandchildren, I met my fiancé in 2017 and were due to be married (covid has cancelled our wedding). My fiancé is an older dad at 51 with a daughter who’s 5, in first year he’d have his daughter every weekend but December 2018 he had his daughter over xmas and the birth mum did a disappearing act so he went for custody and got it.

In that time I became very ill with a brain tumour and needed surgery (I’m going blind in one eye). Well because I don’t work I look after his daughter full time, I do absolutely everything for her including buying her clothes, taking her on trips/holidays, I’m the one that puts consequences and boundaries in place, I’m consistent in her life although I really struggle due to my illness.

Now Birth mum is back on scene, in and out of my step daughters life, is sofa surfing after being evicted, is on drugs and puts herself on sex selling sites (even used her kid as bait ), she has no overnight visitation.

She hadn’t seen her kid since 5th March but saw her last Saturday which she was gonna cancel as we had a rain shower but my fiancé told her no, this weekend her excuse is it’s to hot so won’t be seeing her kid.

On top of this the stupid woman has made some serious allegations to social services about me, apparently My step daughter screams when she’s got to go home, because I abuse her, I slap her, the list goes on, she’s 5 years old and is constantly lying. Neither parents can co-parent but we’ve been told we will be interviewed under caution because step daughter is so afraid of us ( I can prove otherwise). There is a court order and mum has never ever stuck to it plus she fills her kids head with all sorts like calling her and me a f**king rat, she’s gonna kill me, to wreck the kids bedroom cause I decorated it, again it goes on.

I love my fiancé so much but I’m absolutely fed up of having to prove myself to people, this is hampering my recovery, if I’d known it was gonna be like this I’d have ran in opposite direction. So I’m asking anyone that’s step parented how the heck do you deal with a very bitter mum ?

There’s so much to this saga but I don’t wanna go on and on .