What if you run out of money in the middle of an estate suit?

If the estate was sued, or you were sued as the estate's executor, personal representative, or administrator (i.e. for things you did only in your official role, after being appointed or approved for it by the courts, in managing the estate), the estate should pay you legal bills: defending itself, or defending its executor, etc. for doing his/her job, is a legitimate estate expense. In this case, you should be able to legally tap estate funds to pay for the litigation, and you should be able to submit a written motion to the court to have the estate pay you back for any litigation expenses incurred.
HOWEVER, if you are being sued for things you did pre-estate, i.e. as your mother-in-law's POA (for example, that you took, hide, used for yourself, or wasted her money while she was still alive), or did after your mother-in-law's passing but *not* as part of your official duties as the appointed executor, personal representative, or adminstrator (e.g. if after her death, it is alleged that you used your access to her accounts or property to take her money or belongings), then any legal costs, etc. are not estate expenses: they do not come out of the operation of the estate or the activities of the estate's appointed representative in doing his/her job. Rather, defending yourself from a suit about you did not in an estate capacity is your own expense, and you have to pay it or else represent yourself in court.