My 2 1/2 year old niece is constantly taking attention away from my 1 1/2 year old daughter. What do I do?

I have been babysitting my niece since my daughter was 3 months old. She is used to being around me and my daughter, therefore, always getting only half of my attentions but it seems lately, since my daughter has been walking and talking (she is 18mo now), her cousin is iffy about her. She's not exactly mean but she's not exactly nice either. I watch another little girl who is only 2 months older than my duaghter and my niece gets along great with her. She holds her hands, asks her to "play" with her things like that, which, I might mention she NEVER does with her cousin. I try not to let it bother me but then she's also starting to act like a baby sometimes, or she'll go to play (sometimes she;ll run to get there first) with a toy as soon as my daughter looks at, or she'll take a toy my daughter was playing with if she drops it or sets it down for a second then parade it in front of her, it seems like almost to get her to take it from her (because no one is allowed to grab toys away from anyone in the house) so I will have to give my kid a time out. Which is another thing....she loves when my daughter gets in trouble (meaning I scold her or put her into timeout) and also loves to try and discipline and yell at her, which I always tell her to stop doing. I love my niece but its getting hard to keep watching her everyday when I sometimes want to just pay special attention to my daughter for even a minute and her cousin suddenly has to pee pee (shes starting potty training) or will suddenly need water or jump in front of my daughter and try to take the attention off of her. Ps- i do pay special attention to my niece as well. Usually she gets 20 minutes of just her & me everyday after the younger ones start their naps before she lays down for hers and besides that I don't know what else to do but its driving me crazy and I need advice on how to get it to stop. Please?! :)