You and your sister are not at risk under most circumstances here; rest a bit easier.
There are issues however, the most important of which is if the house is still in your Mother's name, whoever acted as the personal representative didn't finish probating the estate, and creditors (of your Mother) may be able to take the house from your brother if notices were not published etc.
Who was supposed to be handling your Mother's affairs after death (the "personal representative" named in the will, or the person who filed probate as PR if there was no will)? THAT person may have not completed things properly and in some cases that can lead to liability.
You should consult with a local probate attorney to get this resolved once and for all.