It's a long-term ploy. She's hoping to manipulate either Eli or Bucky into doing the honorable thing and taking their shirts off to lend to a poor, freezing archer. After all, she hasn't got any brainwashed-assassin-training or hereditary super-soldier-ness; either of those two could get along without a shirt on much better than she could, right? So they should lend her theirs. Obviously. If they're cold after all, she's willing to let either or both cuddle up next to her to keep warm, because that's just the kind of selfless hero Kate is.