Maybe if you get it all down on paper in a concise but complete outline, it will firstly let you clarify in your head exactly what the problem is and how you feel about each item. Secondly, be able to discuss in detail how you feel with Mr Hoof Hearted and get his input.
After you, or is that y'all?, decide on what must be done, you can discuss your view/position/plan, with SIL & BIL as a united front. No appeal, that's the way it must be. If they are offended, then they are offended by you both.
Best for all, methinks.....good luck.
