Your answer is in your last paragraph: you have different values. She has been telling you for years that she's not all that interested in some other people in the family.
I wonder if a few things have been said too harshly. Maybe you didn't mean it this way, but you said "I put her as a bridesmaid." Most people don't "put" anyone there - they invite and explain why the prospective bridesmaid would enhance your day and make you happy because of her wonderful qualities. You say she "seemed interested" but it's not clear that you really talked openly with each other. You seem to work more on "rules" - such as when you say that it's a rule that weddings and funerals are mandatory family functions. No one likes to be told there is only one set of rules for life, and it's yours.
You have another assumption that stay-at-home moms don't work, and one that people who work at home don't actually work. I have done both and you couldn't be more wrong. So perhaps your feelings are coming off to her as disrespect, even if you don't intend them that way. So maybe she wasn't "being dramatic" but was already hurt, and maybe her "excuse after excuse" (while extremely frustrating to you) were her many ways of telling you no.
In reading your responses to many questions on Mamapedia, I observed that you have an extremely direct style. That doesn't sit well with a lot of people, so maybe you are putting people off either intentionally or unintentionally. Some people have very different communication styles, and perhaps you and your sister are in this category. Maybe that's why she made a bunch of excuses, because she's not the direct sort of person to say, "Sorry. No." If your sister doesn't feel you'll listen to her or respect her position, she may just say no to things. It's even possible that you have hurt her in the past just as she is hurting you now. You can work on that if you want to, but maybe this isn't the time when she seems to have made her decision and when you're so emotional and have a wedding coming up that obviously is important to you.
Re the will - what's the point? You don't trust her. Are you planning to punish her or try to blackmail her by saying, "if you don't come, I'll disinherit you"?? Besides, you are getting married. Your husband will be your next of kin and should be in your will. If you want to give him instructions to take care of your parents, do that. Or, set up a trust with a bank so there is a trustee to pay some of your parents' expenses and nothing else.
You can't control everything, only your reaction to things. Get back to focusing on your wedding, who is coming and who loves/supports you. Don't miss out on those friendships because you are focused on one that isn't working.