Here's the kicker: When the parents pass, you will be cut out of the inheritance. BIL will get it all because "you don't need it but BIL does." I've seen it too many times.
(Not a psychologist) but there seems to be a codependency with DH and parents. These things rarely end well. One of my direct reports had to w*rk to 65 because her second husband kept bailing out (figuratively and literally) his son from previous marriage. Eventually, the son committed a federal felony and has ended up in prison for many years. It's (unfortunately) the best thing that has happened to everyone but the "evil" son.
Not my place to say it, but I would tell DH to stop the money flow. Believe me, with the assets you mention, they will figure it all out and survive. You are NOT the wicked witch of ANY compass point! YMMV