What will my child call their sisters mother?

Ok, so let me start off by saying that we are not poly haha. Long story short, I moved to another state and married a friend from high-school. His ex wife and I became good friends and now we are splitting a household with her and her new husband. They live upstairs, we live downstairs and we take turns with my husband's daughter. Also we are only there about two weeks out of the month, due to my husband's work.

We recently found out we are expecting and we want the baby to have titles for the daughter's mother and her husband. But we have no idea what they should be called.

Any suggestions would be helpful!