Unless you are the owner of the property you are not the person who needs to hire an attorney. If your mother is on title to the property, then she needs to hire a lawyer who is experienced in eviction cases.
Generally on owner of a property (owner of record – see the deed) can evict a non-owner (tenant). Even is not rent was ever paid, the owner can evict the tenant (usually a 30-day notice is needs to be served first). I suggest your mom sees an attorney that practices real estate law in the area where the property is located. Good Luck.
It appears the siblings may have a partnership among themselves. If they do have a partnership then you may have to do an accounting, partnership, or dissolution of the partnership. Depending upon the agreement, the husband may be a partner. Contact an attorney and have a frank discussion with them.