£500k!!! oh dear a bit of green eye I think. I note you haven't noticed any favouritism between your children and the SIL's child, why are you keeping cousins apart. I guess your husband and his siblings are all loved but one of them needs more care, that's life. Does it all come down to money?
How to Keep Living at Home Longer
When a political leader lies on their CV - can you trust them?


Nothing really to do with the in laws behaving in an unacceptable or unreasonable way; you're just jealous of your sister in law; the money they've given to her and the time they spend with her. MercuryQueen is right: It's time to change the situation.