Without knowing more details, there could be some rationale on the SD's part. Did they come more or less directly to the party on their return from their weekend? If so, they probably didn't think of gifts while in their pre-weekend rush, and may not have had time afterward. Even so, showing up to honor their dad was way better than not showing up because they didn't have gifts.
Even if they have no such excuse, it's good to remember that gifting is voluntary, or else it's not a gift (maybe more like an obligation, or a tax). I sure don't want people to give me gifts because they have to. And I have, some years, not given someone a gift because I never found the "right" thing, and didn't want to just give some obligatory token.
If your sister has good relationships with these daughters, that's a lot, and worth preserving. Judgements and expectations tend to get in the way of good relationships, so if I were her, I'd leave it to her husband to let his pleasure or displeasure be known to his daughters.