The daughter of a friend of mine got married last year and the invitation said "black tie". I immediately thought, tux & long dress. Of course, neither my husband nor I own those clothes so I called a mutual friend who I figured would know. I didn't want to call the friend right away because I didn't want to sound, well, as if I were complaining! So exactly!
Anyway, the other friend did have the scoop, she said, men could wear suits and women didn't need long dresses but that (in her words) "black tie is the new way of saying for men to wear suits and women to wear a dress".
Now, if I'd had to go out and purchase/rent the clothes, I'd have done so. I might have grumbled to mutual friend, but to MOB I'd not have said a word! So I agree that your SIL's wording definitely comes off as whiny and critical. I can sympathize about her concern for the cost, but what I can't sympathize with is her throwing that tidbit of information your way.
So yeah, fine to ask, "did you really mean tux & long dress or are dark suit and short dress ok?" but not ok to say "cuz if you mean tux & long dress, it means we have to spend money we didn't want to spend."