I agree with being a bit less militant while you're still staying with your parents. You will have full control over your own life choices soon enough. I mean, if you go vegan you don't expect everyone else in the family to eat that way too? Your sister needed to come with you, she doesn't cycle, cycling and taking the car were the only options. If those were the facts, I'd take the car too.
My SO is gradually getting into this cycling thing, but what's a reasonable distance for me is still very much beyond the comfort zone for her. We take that into account when we plan for holidays, for example. Sometimes we cycle, sometimes we rent a car. We've come a long way from not cycling together at all.
--J