Our DD8 has issues like this all the time. What works best is stepping in right away and giving her a choice of playing appropriately (with a clear explanation, e.g., kick only as far as the younger kids can kick; speak nicely to your sister) or leaving the situation or getting a consequence (e.g., a time out; not being allowed to join the kids again or for a day or so; and additionally losing a privilege or dessert if she argues). Unfortunately with our DD it takes a long time for correction to have a positive effect on her behavior. Explaining that kids won't want to play with her if she acts this way, although she understands it in theory, is not enough to change her behavior in the moment. So having them play in your own backyard where you can reinforce appropriate behavior when you see a problem is actually a good situation for you to help your kid with their social skills, IMO. Good luck!
