Thank you for contacting JustAnswer.
I am sorry to hear about the problems you are having with your sister-in-law. It may be helpful for you to suggest play dates where each adult takes turns having the girls at each of your houses or taking them to do activities together. You might suggest that this might be nice to take turns this way and give you and your sister-in-law some time without the children so that each of you can have the opportunity to accomplish other things. In addition, you could consider having the girls take a class together where you could each take turns in transporting them.
I hope this answer is helpful. Please let me know if I can clarify further.
One of the best ways to deal with this may be in working to not display emotion about this. Remaining matter of fact and simply setting limits of how many play dates per week are allowed can allow you to focus on the rule rather than the feelings related to your sister-in-law. You could also encourage your daughter to get involved in activities that would also take her in different directions and expose to her new people. In addition, you might want to get your husband involved in arranging the play dates, or helping to set limits about them. Please let me know if I can help further.
My husband is on her sister's side mostly. She loves our daughter and his niece to get together all the time. This is only my problem.
That may allow you to enlist your husband in helping out with talking to his sister and doing some of the arranging of the play dates. In addition, it may be helpful to write your sister-in-law some angry letters (that you do not send her but instead shred). These letters can be quite helpful in venting your anger to make it more easy to deal with her. In the angry letters you can say anything you want. Just be sure to shred them as soon as you are done so that no one finds them.
I hope this is helpful.