After the house is sold, if it sells at auction, it will be up to the buyer what happens next. They may choose to continue renting or they could choose to evict. I don't know what the local laws are for this, that'w where you attorney can best advise you on what a buyer could or couldn't do.
If it doesn't sell at auction, then it is up to the foreclosing bank to decide to rent or evict. Again, this is based on the local laws that pertain.
To summarize, find out what you can about the forclosure and the likely timing, as well as seeking legal advice on what your rights are when that process runs its course.
IAre you or your sister in a position to buy the main house and rent it out? It would be the best way to control the situation.