Reading between the lines here, so sorry if I am wrong… also, ignoring her interactions with others, it’s not yours to police, yup, even the gran situation.
Your younger sister has always come to you with her problems and being the bigger sister you took care of her. This has been going on since she was a kid, so for 20 years.
As she got older the initial dynamic continued, she had a problem, reached out you dropped everything and became what she needed.
Meanwhile you start to resent her behavior with her interactions expecting her to behave another way, but most likely didn’t have a conversation with her, I’m guessing you made a snarky comment here and there, that she ignored (or simply didn’t get).
Then, out of nowhere you blow up… and start listing all of the past transgressions when she should have behaved differently.
This is the thing, at all of those times you made a choice too, you put her needs ahead of yours, she did not do that. Can you blame her for acting how you have allowed her to act for the past 20 years?
Is she selfish or or does she put herself first? Seems that you may envy her ability to say no when she doesn’t want to do something, but you think morally she should do it regardless,because that is what you would do.
Human beings do not all behave the same way. Are you projecting expectations of her behavior based on what you would do? If so, stop, that leads to nothing but frustration. Accept her for who she is, take control of your behavior.
She is now a quasi adult, so you don’t have to drop things to address her issues, you can say no! Be explicit, if something is bugging you speak up, but allow her the same courtesy, she could be experiencing you as domineering, always right somewhat critical big sis?
Go to Paris, you will regret not having that experience with you mom!
eYou want her to behave like an adult, it’s about time you did the same here.
Talk to her!
Best of luck!