Boyfriends immature am i wrong?

So ive been noticing a trend in my boyfriend being really immature. I would like to know if im being too critical or if this would bother any one else as well. Ive noticed him extremely immature especially when hes with his sister. I have other examples but this is one i just saw today. My boyfriends 25 and his sister is 23. Im 22. We were making breakfast this morning and his sister complains theres not enough toilet paper because my bfs always blowing his nose with nothing coming out lol. So to piss her off as shes about to go use the restroom, he sticks his foot and half his body in the doorway and complains "i need to blow my nose to show you stuff actually comes out" and they are in this screaming match, her telling him "get out i need to pee" and him yelling back "open it i need to blow.my nose" then him yelling "i cant get my foot out". She opens the door and he walks away all pissed. She comes out and he blows his nose in it and shows her and then jokes about putting it on her pillow. His dad is yelling at them " grow up". Is this behavior that a 25/23 year old should not be doing?