I feel so bad even thinking this....I have no proof, just that my sister in law quit her job after they moved in with him. And my brother in law has bought a new truck and boat since living with him. He is on disability and he always had money trouble before.
Can your spouse ask his/her brother to explain the situation? For example, did BIL pay cash for those purchases, or is he making monthly payments, perhaps with the money he formerly spend on housing? Don't ask questions in an accusatory way -- don't risk the relationship for perhaps nothing. But, "Gee Bro, how'd you afford a new boat and a truck in the same year? Wish I could figure out how to do that!" might be neutral enough.
How about grandpa? Can he be asked about the financial situation?
You can report suspected theft to APS, but I don't suggest that without giving BIL a chance to explain and/or without more evidence than you have.