Bf with a daughter from another.

I've been with this man this his daughter has been born.. his ex and him separated before she was born.. i have held her in my arms when she was only a couple weeks old.. she is now 3.5 years old....she is in the terribly three stage.. 

anyhow her mother as far as I know let's her daughter sleep in her bed at night ( every night)so when she comes to our house for her sleep overs she kicks and screams and puts up a huge fuss to sleep in our bed.. eventually and usually he always gets her to sleep in her own big girl bed in her own room at our house...but sometimes he lets her get away with sleeping in our bed and Im stuck with sleeping either in her bed or on th couch..( I don't sleep in the sad bed with them it is my choice because she kicks and rolls around and my sleep ends up being the worst sleep ever) ... so that leads to me getting mad when I tell her no she needs to sleep in her bed and he gets mad when he sees that I'm mad or silent towards to him on the morning...I have explained myself to him and that I believe she should always sleep in her bed.. why do I always feel like such a cunt when I express my feelings and thoughts about his daughter and the rules in my own house ??? 

I feel like the outcast