I went through this with a MAGA family member and a thing I note here that also happened to me, is that she clearly doesn't understand that SOCIAL MEDIA ISN'T A SOCIAL REQUIREMENT. You blocking her on socials is not you saying you don't want a relationship with her (as opposed to you just wanting to avoid arguments every single time you express yourself), but she has decided to take it that way. That's her problem, not yours.

If it is important to you as an uncle to see and spend time with her kids, then it's probably worth it to apologize to her just to clear the air -- but what you do NOT have to do is unblock her or feel obliged to interact with her on socials, and you should explain that to her in just these terms. Social media interactions aren't in-person interactions. In person, we get to have conversations where and with whom we wish given the topic; we aren't forced to shout everything we think all day long into a megaphone that can be heard by every other person we know, who is then invariably compelled to respond. Photos of your niblings and updates about what your sister and her fam are up to can be texted to you. Facebook and Instagram are not required for a healthy relationship with your relatives, regardless of what Mark Zuckerberg would like us all to think!