Your sister is a bitch, she just wants it for free/cheap and feels entitled because she is your sister.

Watch out guys..... There's a bridezilla on the loose! All the jokes aside OP, as much as your sister is being pretty rude about all this, i do hope it all works out for everyone involved and that the wedding comes out amazing ;)

I mean, you are technically being selfish by preferring to do what makes you happy rather than helping your sister with what makes her happy. Buuut that level of selfishness is completely normal and acceptable. Your sister on the other hand is making selfish demands that is on a completely different scale from your selfish actions. Just explain your situation to them, if they can't understand you get to choose between making them sad/upset or ruining your own life and future. I know what I'd pick.

Sounds like your mom (and, following her example, your sister) have learned that pushing the selfish button is an easy way to get whatever they want. They aren't thinking through what they're saying at this point, they aren't even morally judging you, it's just become their magic phrase to get their way.

I hate it when moms join their side with relentless guilt tripping. I had to stay up all night doing my sister's homework in school despite being in University myself. eventually I got fed up with doing free slave Labor and I told my sis that her research papers would cost money and I'd only start on them once the money was deposited in full to my bank acct. I sent her links to PayPal. she retreated saying she had papers to write. she learned not to wait to the last minute. taught her life lessons. stand your ground. say no or set your terms. the guilt trip bullying will cease.

Just who does she think she is?

I'm confused as to why she can't make her own.