Crazy mil

Okay sooo long story short

My fiancé and myself and our son are currently living with his mom my fiancé also has a daughter from a previous relationship who stays with us pretty much full time because her mom is in and out of her life and cares more about partying right now instead of being there to take care of her daughter at all! Which we would rather have her living with us right now regardless.

So we’ve pretty much had full custody with an occasion one or twice a month sleepover at her moms. Since this started happening I’ve pretty much been taking care of her this whole past year since school started which means getting her ready, taking her to school, making sure I’m home to get her off the bus and then we do homework and I start dinner and my mil comes home from work and my step daughter pretty much hangs out with her until she goes to bed. My mil and my step daughter have a very close relationship and from what she’s told me shes pretty much been filling in as the mother for her since she was born at 30 weeks premature. She’s very tiny for her age and has some mild health conditions that she’s had since birth. She’s still a very normal child and for the most part lives a normal life. She was a very tiny and fragile baby and that grew with her as she got older. Still tiny and still fragile but, because of this my mil treats her like she never does anything wrong EVER and if I try to discipline her I will get ignored for the rest of the night. This has gotten so bad that it’s come to this little 9 year old girl completely controlling our house. My mil let’s this little girl disrespect everybody in the house, and if you do yell at her she goes and screams and throws a complete temper tantrum. Throwing all of her toys, her 2 year old brothers toys. I’m so sick of it. I feel like I’m only good enough to watch her during the day and not good enough to be able to actually discipline her.

I don’t know this was longer than expect please help.