trustee per stirpes...to Mom's estate

No, I don't think so. According to your post, you are supposed to "distribute the assets of that trust to the decedents of her Dad, per stirpes." So that's what you're supposed to do (although I'm sure you mean "descendents," not "decedents" -- BIG difference). Practically, though, there might not be any difference. If the children of your wife's parents are all still alive, and presuming that your mother-in-law divided her estate equally among her children, then the remainder of the trust would get divided equally among those children, regardless of whether it passed directly to them or through your mother-in-law's estate.