The fact that she has $90k in debt for degrees that she apparently isn't using is a red flag. Her (incomplete) track record you have posted makes her sound more like a perpetual student. I would hesitate to cosign.

Will she actually be able to do the type of law work she wants with 4 kids at home? The law market is saturated, and there is no guarantee of work after graduation. Will she be able to keep up with the hell of getting her feet wet at a big firm while still being able to pay attention to her family. $230k is a lot of potential burden on you so that your sister can chase a romantic dream that few ever find.