Also, reference, while you have a salient point there, don't forget that the other strongest mention of memory in the film comes from the memory dialogue between Cloud and Sephy. Additionally, Sakurai, Cloud's VA, says he believes the memory refers to Sephiroth, for as much weight as his statements are worth.
Yeah could be that too.
The issue really is just with regards to what had
Tifa meant with "a memory". We as viewers could draw numerous (Aerith, Zack, Sephy, etc) assumptions via Cloud, being on the receiving end of the question. But it is harder to draw assumptions from the question's origin [Tifa]. What did she really mean by that? What is she, as a character, focused on?
One could say, and I say this hypothetically, that she was addressing the memory of Aerith's death based on the context of the conversation in which she asked the question.
Her family is falling apart; the kids were taken and Cloud is reluctant to do anything about it. He mentioned it himself: "I can't protect anybody. Not my family, not my friends, nobody". If Cloud is expressing his inability to protect, I think it's natural that Tifa would think of the past incident where Cloud had failed to protect somebody in which she was present and also affected by (Aerith's death). She could be "jealous", and I use that word loosly, or insecure of her importance to Cloud if he's prioritizing Aerith's memory (the memory that is hindering Cloud from rescuing the kids, which then prompts Tifa to drop the bomb and ask the question "a memory or us?").
So in favour of the idea Tifa adressing Aerith's death:
a) she knew about that particular memory (she was there), more so than the other possibilities with regards to which memory
b) linking that particular memory to the current situation (Cloud's guilt of his inability to protect) and there for questioning her [their] importance to him.
^ That's all hypothetical. I'm just saying it could be a reason.
But there really are many conclusions to be drawn and assumed (like you said, you can throw Seph in there) with regards to what she was in fact referring to. So... I'm still sitting on the fence (a fence with many sides apparantly) and think it has a mix of Tifa's insecurities along side Cloud's guilt issues.
In response to the LT jealousy claim, I don't think it's entirely because of jealousy. Otherwise, she would've just said "a memory or me?" She was wasn't only speaking of herself, but also Denzel and Marlene.