Did Sigourney Weaver really deserve an Oscar nomination for Aliens?

Yes.

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I think she was good. Though I haven't seen the other nominees so I can't say. At least she won a Golden Globe for Gorillas in the mist.

To give a more in-depth answer than my previous response, this was a fairly lengthy script for its genre that featured Weaver in almost every scene, running her through an entire gamut of emotions both intense and subtle, along with grueling physical work, and required her to maintain a gradually developing character arc throughout the whole thing (and remember that the film was shot out of sequence!). She has to carry the entire movie on her shoulders and spin a lot of plates while doing it. That's a tall order, but she passes with such flying colors you completely overlook the "work" she's doing while watching.

And then you add on that she absolutely nails being a credible, serious action heroine (while pulling off the "blue collar everyman action hero" thing a year before Bruce Willis!) which was a hard thing for people to even envision at the time ("female action hero" was basically silly stuff like Charlie's Angels) and I have to say yeah, Sigourney Weaver fully earned that Oscar nom.

It might have been to make up for not giving her a nomination for the original?

In my opinion, yes.

yes. And she should've won