My mother's sister is married to my father's brother so how can I link them to indicate they are sisters

Answer 1

You can delete one of them and re-add her as a sister of the other, then click "add partner" and "partner with person already on tree". Then you can link her to either your father or your father's brother (depending on which sister you are editing). Or you can look at this link for another option which I find easier: http://answers.familyecho.com/4064/members-family-siblings-without-deleting-them-adding-them

I hope this is helpful!