I'd send some money as well (because if her clothing is too small, her underclothing will be, too, poor kid), but with your ESIL so inclined to re-appropriate funds, I wouldn't send a check, but a gift card to a place that mostly only sells clothing. Old Navy or Kohl's would probably be a good option.
If the shirts/sweaters or skirts that you send are a bit too large, she can probably still wear them, but that is harder to do with pants because of the length. I'd also consider sending a kit of sewing basics that she can use to take a tuck here or there if needed. (Even if she doesn't really know how to sew, necessity is the mother of invention, and the odds are she will speak to her friends and find her own way to alter them in a way that works for her needs.)
Also, tell your brother that unless he is paying the support through a court-verified holding account, he needs to be getting receipts for those advance payments that state that they were advances on regular support payments, because it will be very easy for her to claim that those payments were extra gifts for DD's needed clothing, and that the regular support in the following months was not paid.
As to who puts a kid in the middle of support discussions, I'm guessing that might not really have been the case. It may be that she overheard her mother talking to someone about the rent situation. As a kid she can't really earn money of her own very easily, but she knows that her dad pays her mom support, so that could just be DN trying on her own to help keep the roof overhead.