How can I raise a well behaved child around rowdy and misbehaved children?

My husband and I currently live with my mother and her two little children/my little siblings (3 & 5 year olds) and they are really rowdy and rude. They'll tell my mother "You better go get me some milk" and things like that and disobey when you tell them "no" or "to stop". They're good kids they just haven't been disciplined right in my opinion (timeouts, etc) and now I'm pregnant with a baby of my own (I'm 24 years old) but my concern is that since my baby will be growing alongside my siblings that they'll pick up that behavior (especially since I'm having a girl and one of my siblings is a girl). Is there any way I can raise my child to be well behaved and polite even around my siblings?