Tenancy in common in your favour? I don't understand that bit.
Regardless, as joint legal owner she can apply to the court for an Order of Division and Sale. The courts only rarely refuse, and that is when there is a very good reason why the house should not be sold, for example it has had extensive adaptations to accommodate the disable resident.
Having said that, if it is in negative equity there is no equity to transfer and she will be equally responsible for any debt.