"We receive lots of adult clothing and young kids but seldom items for teens."
This called up an entertaining image. "Here's three shirts, two pairs of pants and a toddler. Glad we could help."
"Personally, I would donate so that someone gets the benefit. However, doing so gives the in-laws the impression that you kept them - opening that door a crack. So they will continue to give the gifts and may even, as the children get older, contact them directly about the gifts. So, in this case, I'd send them back."
Truth be told, they'll act like that no matter what. When dealing with the level of toxicity that requires cutting someone off, you're outside the realm of sensibility. If they want to believe the door is open, they will despite any and all evidence to the contrary. As evidence of that, I present the presents themselves that deadbody fears will be left on the stoop after they were told not to contact the family. The only way to deal with them is not to play the game at all, in any form. Sending back "gifts" forced on them after they specifically told her parents not to contact them is interaction, and the deadbodys have decided that they want no interaction, so why would they let the cut off relatives' reaction dictate that they must interact?