Please may I share you what my sister did to help me when my son died?
She completely got it when I said "I am unable to speak." And she undertook to make every single phone call (Other than to DD30) that had to be made. And then she marshalled every single one of Jim's cousins to attend the memorial, including his closest cousin who was en route home from a visit to his wife's family in Russia. Every one! And she handled hotel arrangements and transport and just all kids of stuff, and she couldn't afford a bit of it.
There's more, but there is a start. let your brother and SIL say what they need right now and do whatever it takes to get it done. Simple, but not easy.