Can't your sister family sit in a car? Is it a must?

IMO, just for 3 months, you really risk losing money (unless you get a super buy from an individual on craigslist, and you are good at dealing and not bothered to go through registration and later selling). If your sister will not sit in your car, i.e, her family MUST have a van otherwise they will not come, and you MUST have them here, well, consider to buy one and continue to drive it for your own transportation. Alternatively, lease a van for three months...

I surely don't know exactly the reasons but just want to help you to square off your frustrations down the road...