How to do it is simple:
Python:
label whatever:
$ girlIs = renpy.input( "What is she for you ?" )
if not girlIs:
$ girlIs = "roommate"
"Noted, the girls is your [girlIs]."
But having a story that will make sense while doing is it something extremely difficult.
The reason why a roommate would refuse to be sexually involved with the MC isn't the reason why a sister would refuse. Therefore you'll end like all the game doing this, with dialogs as stupids as, "no, we can't have sex, I'm your roommate, it would be so wrong."
Letting the player define the relationship is
not how one should deal with Patreon ban regarding incest. There's many thread explaining how to do it correctly,
this one being probably the most complete.