Quitclaim Deed - Conventional Mortgage Question

[Donnie Maguire]

I have a unique situation that I cannot seem to find a very clear answer for, hopefully the BP community can assist.

My wife and I got married last September and now live together in a house we purchased together. Before that, she was living with her sister in a house they purchased together. Her sister still lives in the house with a roommate so my wife's portion is covered. Her sister is engaged and will be married in the fall and will be moving out of the house into a new one with her soon to be husband. 

My wife and I currently have 2 single family long term rentals and would like to add this to the porfolio due to their 3% mortgage and high equity. Her sister has agreed to us paying out her portion of the equity and having us use this as a rental. 

I know we can have a lawyer draft up a quit claim deed to get her sister off of the deed, but I have head this is similar to when people transfer ownership to LLC and could pull up the due on sale clause?

Is there anyway to get her sister off the mortgage without refinaning? 

Just trying to figure out our options here, would love to keep the interest rate if possible for obvious reasons. Curious if anyone that has dealt with a divorce/separation scenario that might be teh most applicable.

Thanks!