What to do about a bratty niece?

I watch my niece 5 days a week for about 10 hours each day. I do all sorts of things with her during the day. We read, do activities, swim, play outside. She tends to be a little on the bratty side because my sis and bro in law coddle her. Once in a while she will cry obnoxiously when they leave. When they are not here, she is great. Listens to what I say and behaves. However, as soon as they get home, she is a major brat. to ME. whines at me, cries when I tell her not to put something in her mouth, slams the door on me when I leave, yells at me out of the blue for NO reason. My sis in law doesn't do anything about it? What am I to do. I hang out with them a lot. Everytime we're all together, my niece acts like that to me. What do I need to do?