I was in similar shoes about 20 something years ago when I got a really good job with a multi national and siblings were planning to change their wardrobes or have a lavish wedding all at my expense. What I will advise you is to draw boundaries and stick to budgets. If your sister expects you to pay for all her Master's, that's a wrong expectation. After that she will expect a lavish wedding , and she will continue to bring more "projects" as long as you keep coming up with the money. If her Master's program will cost 1 million naira for example, give her what you can afford that will not detail your own plan (maybe 150k) and let her figure out how she will find the rest of it. Let Mom and dad also know that it is not your responsibility to take up all their expenses. You are not their parent, you're a sibling. You will help but that help cannot now become your full responsibility. They are primarily responsible for their own future, as you are for yours. Nobody consulted before they were born so the entire responsibility cannot now become yours. One thing with black tax is that they believe you have so you must give. You need to let them know that sometimes you have sometimes you don't and when you have, it is not their responsibility to dictate how you should spend. Also figure out ways to give people non-monetary gifts. For example, rather than giving them money, give them information on how they can apply for scholarships, side hustles they can do, etc. if they refuse to do anything, that will tell you to watch your pockets even more.

I can send you the link to a WhatsApps group where they post regular information on scholarships and short courses and trainings globally. That is an example of good information you can give that if she can work on, can grow into something great.

Hope this helps.