Veterans

If his Rep or one of his Sens is on the Veterans Affairs Committee, that Sen would be the one to go with as they oversee the VA.

Understand that this is ordinary constituent service, and unless the Sen or Rep is invested in your issue, he or she is just going to pass on your complaint to the DVA Secretary and ask for an answer to it.

Because it's a Congressional inquiry it will get a quick response, but it won't necessarily be helpful or solve anything. It's up to your SIL (hopefully with competent assistance from his claims rep) to frame the complaint in a way that focuses on where the VA has ignored evidence or otherwise acted egregiously in handling the claim.

Good luck!