Need some advice on baby sitting family

So girls I need some advice. I watch my niece and nephew about four days out of the week. They are great babies being only two and one. However my nephew throws fits when he isn't held. I'm 28 weeks pregnant with an 8 month old son who doesn't even do that. I've asked my sister about it and she says she holds him because he wants it all the time. She says he is mommies little boy and he just smiles. Don't get me wrong I love my nephew with ALL of my heart as much as I love my son. I also noticed how is much more disciplined with my niece and let's my nephew get away with things that she wouldn't let my niece get away with. I'd do anything for him but I CAN NOT hold him all the time. And we all know when one child screams crying...the others Cry. What do I do? Please understand that I'm not trying to bad talk my sister. I just want her to realize I can't do what she does on a daily basis. Please no bad talking.