Firstly you need to sit down with your DH and explain very clearly why this dynamic has to be stopped.
Yes it is about power - and they seem to think that if they bring money into your relationship, then that buys them the right to decide to marginalise you... within your own family.
So you tell your DH - the choices are this. Either you stop accepting money from them, a flat no. Or, you talk to them, make it 100% clear that gifts of money are coming to HIS FAMILY, not just him, and if they make a point of snubbing his wife (and therefore his choices, his family set up) via money, then it can no longer be accepted. (As an absolute minimum here there should be a joint bank account set up and they should be told that larger cheques will ONLY be paid in there. They write YOUR name on that cheque, or it gets handed back.
Or, third option. He does nothing, and their attitude corrodes your relationship with him (because he does not have your back and is not prepared to put your feelings above theirs) and your relationship with them. One day, they'll be looking back at the very distant relationship they have with their son's family and will wonder where it all went a bit wrong.
In the meantime - did MIL always work full time? If not, then she's presumably in exactly the same 'minor category' to you - the little add-on who just works part time and looks after the kids and house, I mean practically a parasite :) Do make sure, then, to apply PIL's standards across the board.
'Yes, driving the car FIL bought for DH today. Sorry about that, I'll clean it when I get home'
'Oh thanks FIL, DC will enjoy that. Don't let me touch it now! Give that money straight to DH.'
Directly to MIL: 'We're so glad that FIL was able to spare DH the money for his kitchen. It's not as nice as FIL's kitchen of course but you can't have it all'