They are not forcing him to do anything. I imagine the reason they are asking about his overseas bank accounts and assets, is for Canada to determine if he would be eligible for low-income assistance programs to help pay for his medical costs. If they determine he has significant assets/money outside of Canada, then they may deem him not eligible and would demand he pay out of pocket for any services not covered by provincial healthcare. It would be entirely up to your parents if they wanted to sell their assets in home country or not.
If they can show their assets/money in home country is not significant (so they are not rich), they may still qualify for having some of his long term medical costs subsidized. I have no idea if this counts against the PR sponsorship undertaking that your sister signed for them.
And on a side note your brother in law sounds like a bit of a jerk.