The potential family catastrophe that my brain went directly to:

What if Sister has some kind of housing instability (either actual or potential)? If so, having an "investment property" in the family in the same state could mean that if/when Sister's housing situation collapses, the logical place for Sister to go would be into LW's (presumably unoccupied or not long-term rented) house.

Which would place LW as a landlord to Sister, thus giving LW power over Sister that would not be good in this kind of family dynamic.

But this is pure fanfic, of course.