why must my sister....

My sister is trying to blame my son for bitting his little sister when he has never bit her. her 5 year old dosnt like my daughter for some reason and picks on her hard. she was laying next to him trying to watch the movie he was in his dad's tablet. he bit her 3 times and left marks. we just noticed them after her late nap. I'm pissed she's trying to pin it on my son. like unsaid before he has never bit her and I would of known if he would have because he looked guilty when asked. But these bit marks are bigger than my sons mouth as well. My sister's son will be 5 on the 9th. My son is 3 and is smaller for his age not much but he is. My nephew has had it put for my daughter sense she could crawl and has picked on her sense and it's only gotten worse. she dosent see it because she thinks he's an angel. mind you she hardly disciplines him when he dose he is being very naughty. when he is she tries to blame my 3 year old and now 17 month old. we hardly see them. once a week is all we see them. I'm going to cut it down to two times cause I'm over her blaming my kids when he son is a little brat. I will no longer watch her kids because she won't bother to watch mine even in an emergamcy. sorry I really needed to rant I don't have anyone to talk to. please no rude comments. I know what I'm doing to solve it.