Have they ever put you above work or anything else important to them? Is your sister always "unavailable" when they want a favor but, somehow, it's not "selfish" when it's her? If the tables were turned, would they take time off work to drive you to an appointment? If it's selfish for you to not take time off work, isn't it equally selfish for your mother to do the same? Has your dad heard of taxis?
The whole thing reeks to me of them wanting you to be the "family servant" - sacrificing your needs and wants for their convenience, such as you taking time off work to be dad's chauffeur because he prefers that to taking (and paying for) a taxi or an uber. But you don't owe them that. You really don't. Really, the key here is that they suceeded at (undeservedly) making you feel guilty (is this something they've instilled in you your whole life?) - because otherwise it doesn't look like they have any leverage. Sure, they can complain - my advice in dealing with unpleasable people is to do what's best for you and give up trying to please them. If they complain that you're selfish, that can get a "I'm sorry you feel that way" and if they berate you , rage, shout, etc then it's time to hang up or walk away.