From the history u know of the little shits damaging your sister's car and now your own, I admire u for giving them another chance, but they don't deserve it. Yeh u might have to spend a little to protect your car by some means or the effort to simply
find a suitable place to park it at (maybe you've got a friend who lives nearby, ask to park your car there, or outside their place?) but your car is no doubt worth the effort and any less stress u could save you and sister from would no doubt go a long way in my opinion
Hope we all helped somewhat, all the best.