hmm...maybe you could try having your husband speak with his mom so he can get a feel for how she sees it. I mean, have him discuss it like "i don't want you spending so much money on the baree as its just another burden like jahez, neither of which we, as a couple, believe in"
this way, the MIL will kinda get the hint that you don't believe in either jahez or baree and won't have false expectations of jahez at the wedding. there will be no let down. Also, she may have an easier time understanding your point of view without getting offended if she knows that you told your own parents the same thing and they agreed without getting offended.