How to get my daughter to be more patient with her little cousins?

My sister's husband's passed away in the spring, and I have been trying to help her as much as I can, which includes hosting her two daughters, ages 7 and 5, for weekend sleepovers about 2 or 3 times a month. My nieces are active and rambunctious, but they aren't bad girls, and they practically worship my daughter, who is 10. But my daughter's early interest in these frequent sleepovers has morphed into annoyance and now hostility. She is constantly enforcing rules on them that no one else cares about, yelling at them for "destroying" (read: leaving out) her stuff, and avoiding/hiding from them.My husband lost his dad when he was 8, so he is infuriated by our daughter's lack of empathy for her cousins. We have both explained to her that we need her to be patient, generous, and kind to her cousins. She cries and apologizes, and then it happens again. She has a younger brother, but he's only 14 months younger, so she is used to sharing space with a kid who, a) doesn't play with her girly toys, and b) is close enough in age that the big kid/little kid dynamic doesn't exist between them.I'm at a loss as to how to handle this. Please advise. via /r/Parenting http://bit.ly/2TgthXq