The sister with the newest profile must be merged into the sister with the oldest profile. However, you must make certain that the two are indeed the same person. (Same birth and death dates and location, same spouses, same children, etc.)
If you would post one of the sister's profile ID, I would be happy to look at them.
At the bottom of the profile is a Matches and Merges section. In the first column is Pending Merges, and below that a link for [initiate a merge]. Click that link and follow the instructions. Do not hesitate to ask for help. WickiTree can be daunting when you first begin.