First and foremost, put any thoughts of minivans and SUVs out of your sister’s head.
Apples to apples, minivan or SUV is waaaaaay more expensive than a sedan. A family of 5 on an extremely tight budget doesn’t need anything more than a sedan.
Look at the ideas posted in the “What would you buy for $10k?” thread, for example pretty decent Mercury Grand Marquis and Impalas with the 3.8L were posted there for $3,000-4,000.