How to deny giving a lift (generally doing favors) to my sister
So, basically my relationship with my sister has always been quite toxic. I've always been a people-pleaser and generally have had trouble saying "no" without an explanation (like a technical excuse why I'm not able to do the favor). My sister, on the other hand, has always been manipulative, lazy and basically trying to outsource everyone around her to be able to do the bare minimum and get away with it. She's also been quite aggressive in her manipulations, reaching even for emotional blackmail etc. She's almost 30, doesn't study and doesn't have a job. Our family sees the problem, but generally my parents try to keep up appearance of a well-adjusted family, so we don't really talk about it. My sister may suspect what I think of her, but I've never told her that straight to her face and she seems to profit off of the fact that we pretend everything is ok.
She has recently found herself a new boyfriend who happens to live in the same direction which I frequently drive on the weekends. She generally avoids public transport - my guess is that it's just easier for her to say that she's scared of public transport and have somebody else drive her, than actually use public transport and pay for it herself.
So she started asking me to give her a lift to about half-way to where her boyfriend lives and then her boyfriend will pick her up from there. The clue is, due to the dynamic of our relationship, I usually tend to avoid her if I can (except family gatherings etc.) and I am definetly not seeing myself taking her with me every other weekend after a work week. How do I deny giving her a lift, when <theoretically> there are no real excuses (like I don't have to go off my usual course, so it technically costs me no additional mileage etc.).