Clashing over parenting differences

My fiancé has a daughter from a previous relationship he is a young parent who is now 26 his daughter is 8. I have been with him 4 years now and we moved in together right away “True love at first sight” thing. So I took to the stepmom/mom of household role very quick and pretty naturally since I am the first born to a single teen mother and helped raise my siblings from a young age and still am. My mom had my brother at the age of 35 had me at 15 so there is a 19 almost 20 year age gap between us. So you can see how I am like a second mom. My mom works nights and I live an hour out of town so M-F I pick him up after work and then take him back to my moms in the morning . Obviously this isn’t forever and is temporary but my brother sometimes lashes out or gives my fiancé a hard time. Given they are incredibly close like father and son close, my brother doesn’t have a dad. My mother allows my fiancé to Discipline him but sometimes I don’t agree with his parenting with both my brother and his daughter. Not necessarily asking for relationship advise or how much “better” your partner is if you can’t relate. Just venting because I know this is a real issue in some relationships. Just wanna know I’m not alone.

Ps. Not leaving my fiancé I have faith we will work this kink out