An adult Sister being Obsessed with her Adult Sister's Children

This question is a bit odd; well to me it is. See I have an older sister, only 2 years older then me and she has 2 kids, ages 6 and 9. She has never wanted Children well at least during the first 10 years that she and her husband where married, and then they planned for these 2. The problem that I am about to mention only happens when they(Sister, Sister and Kids) are Away at their Elder Parents Vacation place. There are arguements that arise when the children are together; one instance was when my daughter was only about 2 and her female cousin was 3; the 2 year old throwing sand, or mud at the other cousin and having the adult sister(2 years older then me) asks why does(my daughter;2) throw things; I simply told her I don't know she is a kid! My daughter even hit her daughter a few times and I chalked this up to them just being kids; I have a older son(now 24) that used to do this and I feel it is normal in childrens behaviors. These fights seemed to have escalated over the years, and only when we are at the vacation place. Most recently last week, being at that place again; my daughter now 8, tells me that her aunt said something to her abd this isn't the only time, she has told me things that my sister has said; but I had to blow them off, because I didn't want to cause problems between my sister and I and my 8 year old daughter didn't want to really tell me, beause she knows about our arguements. Well apparently the kids were playing and there was a rock involved which was my sisters 9 year old daughter and she threw it at my 8 year old, and my daughter yells out to her cousin(name)as if saying watch it. My sister said to my daughter(name)you remember that when you throw things at(name) now this was just during play time and it wasn't like they were fighting(kids)it was just out of play. I got really tired of keeping my mouth shut, and was tired of my sister being able to say things to my daughter freely and I couldn't say anything to her kids. I had emailed my sister to avoid a fight(while being at the vacation place again)Now my sister doesnt want to talk to me or ever see me; She even text my husband telling him that I am self asorbed and selfish and how my first son(24) used to hit his cousin and them my daughter(8) hitting (my sisters daughter, the one who we vacation with)and now my (2) year old hitting her son. Apparently my email had offended her and she felt like I stabbed her in the back(which I don't know what she means)All I know that this behavior that she presents has been going on since 2008(another episode, involving going to Florida)that is a different story though: My sister doesn't like the fact of the hitting, but it is only kids play and no one is getting hurt, except last week while being at the vacation place, my 2 year old somehow hit my sisters 6 year old in the face, and he was really crying and upset about to throw up: We weren't in the room, but I can tell you that I had my son in the room first since he was sick and he was about to go to bed and the 6 year old asked if he could go in the room to watch a dvd that my 2 year old was watching; I was kind of hesitant because my 2 year old was calm and not feeling well; the two girls were playing(8 & 9) and the 6 year old didn't have anything to do, so I said OK; then a short time later the girls went in the room; my 2 year old must have got a second wind or something because he was just bouncing all over the place; going in and out of his play pen, and I saw the 6 year old standing next to his play pen with a smile on his face, as if he liked that my son was going in and out of the play pen, so apparently when my 2 year old came out of the play pen (probably in a flipping kind of way; body hanging over the side) his feet must of came in contact with the 6 year olds face, causing it to be a kicking theory. So, my sister took her kids and went to their camper, and she didn't say a word about it. I guess I just don't understand her and how she can make this my fault? They are kids. Well there is a lot more to this story but that could take awhile.