question about cats

ok so my little family n I are staying with one of my sister and her husband due to not being able to find a place so we're on a waiting list so hopefully we'll have our own place ok so my sister husband has a cat which she doesn't mess with any one but she has been acting strange lately so she comes to us so we can pet her & all of a sudden she either scratch or bites lately she's been doing that to the kids ages 1 1/2-5 my daughter went to put her hand down on a couch didn't see the cat she bite my child then today the cat never goes in into the other bedroom she hasn't been when it use to be my sisters room and for odd reason she goes in n all of a sudden she chasing her tail and pounce like if she was trying to catch something remind you there's nothing there I'm no cat person can some one pls explain