They can sue her estate (遺產). Her auto insurance is obligated to defend her (even when she's dead), because her estate (遺產) is at risk in the lawsuit just as much as her asset at risk if she survived.
However, let's say the wrongful death lawsuit worth 1 million dollars. her auto insurance worth probable only a few tens of thousand (maybe 25K? 50K?) The insurace will likely pay full insured value without a fight.
So the question is, how is the victims' family recover the $950K?
Your assumption is that the mother is fully responsible. In situation like this, a lawyer will look at all the facts and decide they might have to sue the truck driver and his company (because they have higher insurance), or even auto manufacture for defective products. Who knows... Don't hate on lawyer, sometimes you need them for things like this.
