Brother, your sister in law is not your Mahraam and nor you are her Mahraam...so she should not accompany under your guardianship.

In case of Umrah and Hajj, her sister and her daughter who are already accompanying the trip are NOT enough to travel, she should have a male Mahraam with her (brother, son, husband or her son in law or children of her brother/s or sister/s etc).

Hope this clarifies well.

Allah knows better