Ok, so firstly nobody's having a go at you, people are concerned for you, as well as concerned for the 4yr old.
Second, you do have a say.
Frankly, you seem scared of your parents, of going against them, even to the point you won't insist on answers to a situation that affects you, when you have reasonable questions about what's going on.
But if you're legally an adult then you do have a say. You don't have to accept a situation that someone else chooses for you. Yes there could be negative consequences to standing up to someone, but that doesn't mean you shouldn't do it. You have the right to choose what to do with your life, both in the bigger picture of your life's path and the smaller details like confronting people and holding them to account for their actions. Please don't ever feel you have no choice. And recognise that if someone is trying to prevent you exercising your right to choose, that's a bad thing they're doing.
It's good if you can get your sisters involved with this situation, to help in practical ways and as support for you. As for what exactly to say to your parents, maybe they can help with that as they know them better than people on here.
Although, in order to successfully negotiate, you need to have decided what you're negotiating for. Which means you need to decide exactly what you want in this situation. What do you need for your continued studies to be a success and pass the exams etc? What are you willing and able to give to this situation with your nephew? Maybe you could take the role of researching what he needs and informing the others, as you seem to be good at thinking through problems? Maybe there's specific times you'd be available to care for him?
Lastly, it's worth mentioning that you don't have boundaries by asking or expecting other people to 'give' them to you. They can't give you what's already yours. You don't negotiate to be 'allowed' boundaries. You don't need anybody's permission. You have boundaries simply by having them.
This means standing firm and refusing to be manipulated into deviations from your chosen path. It's not the same as being stubborn. You can hear someone's point of view, consider it and maybe decide to change something in your life. Or you can say No that doesn't work for me, it's not acceptable to me, I can't or won't do it. Both are equally valid courses of action. Regardless of how much the other person wants you to do something.
It's not compulsory to give reasons either. Your boundaries don't have to be justified to anyone else, they don't have to be acceptable to anyone else. Other people can feel however they feel about your boundaries, those are their emotions which they're entitled to have. Their emotions are not your responsibility to 'fix', to sort out. You don't have to dismantle your boundaries because someone else is upset about you having them.
This is all stuff to bear in mind when discussing things with your parents.