If you have a debt load owing on  home that you own and want to start gifting or selling her an interest in it, you can probably do so without triggering the due on sales provision in the mortgage. However, you should consult with a real estate attorney to assist you in such an endeavor to go over how to do so in order to not trigger any tax consequences such as a "gift tax" and the due on sales clause of the mortgage.One option is to gift or sell your sister a percentage of the home over time.