My sister and I have never had a great relationship which comes down to her passive-aggressive behavior. For example she'll make snide comments and just have an overall negative attitude but if I say something back all hell breaks loose, it's like I have to walk on eggshells around her.
That was before I cared about politics, but now it turns out we have complete opposite beliefs. She's a college professor and teaches a course called "urban studies", and based on the course description it's largely focused on topics like white privilege and climate change. She's also an abortion clinic escort. She's a vegan and hardcore environmentalist. She's basically a stereotype of a millenial liberal and describes herself as a marxist.
I haven't seen her in 2 years but we have a family reunion coming up and I just know that certain topics might come up at the dinner table like COVID. I'm also against abortion and that's another topic that might come up as well. If I give my opinion and say that I think lockdowns over a flu virus were ridiculous and I didn't get the vaccine I'm sure an argument would break out because she gets angry when someone espouses non-leftist beliefs. Last time we had a family dinner she got angry because I simply asked her and her long-term boyfriend if they were planning to start a family someday.
Now I'm considering whether I should speak my mind within limits if these topics come up or just stay silent to maintain the peace.
Do not stay silent. Stay calm, and do not, under any situation, be provoked to respond emotionally. It is as the saying goes, "Never argue with an idiot. They will only bring you down to their level and beat you with experience.".
This is what trained marxists do.
You did not mention anything else about the family, as in, if others have chosen to stay silent to keep the peace, but in any case, stay calm and the others will start to observe reality; She can't actually defend her views and retorts to emotional manipulation to get you into a state of mind that is easier to deal with from her point of view and experience.
I suggest you do some research on rhetorics and logical fallacies. It is much easier when you understand what she is actually doing, regardless of whether she does it intentionally or not. Trained marxists are taught about this to limit themselves and be better manipulators, but there are always cracks and attempts of manipulation in the heat of the moment. They are aiming for this, as they want you to be in an emotional state where this is "irrelevant", as it blends into the heat of the moment and made more difficult to notice due to emotions.
Also, try and see if you notice the attempts at making everything "simple" and "common sense", it usually starts there in their attempts to label someone as "stupid" and "intolerant".