IMO this situation really should not be a surprise to you. You married your wife knowing about Ted. Parents don’t live forever and so once your FIL passed you had to consider what would happen to Ted once that occurred. I don’t really get why you’re shocked or surprised that your wife wants to take in her brother despite your pushback, and honestly you sound pretty selfish. Does that mean you are required to take care of him or stay in this relationship? No, but I do think worrying about how this will effect you rather than taking a second to consider how your FIL passing effected your wife and her siblings is kinda messed up.

Overall, if you put your wife in a situation where she has to choose you or her brother, I don’t think you should be shocked when she doesn’t pick you. Instead why don’t you help her figure out alternatives. Ways that he can have access to care in a safe environment, and even then you should probably not stay together.