I am sorry that you have cancer and I truly hope you're able to overcome that.
I really don't think anyone here can really weigh in with an honest opinion without having all the facts; which you obviously do not want to share. How can we say you're being paranoid, spiteful, or whatever, when we don't know the circumstance?
Reading the thread however, I would have to side with the family on this one. There is nothing that I can imagine that's happened in the life of your in-laws (BIL in this case), that makes you more angry and hurt than his bio family. I'm going to assume that you've never had a love affair with this guy (which IMO might explain your feelings), but aside from that, I'm just not seeing it and you're obviously not sharing it.
So, given the tiny amount that we know, yes I think you are overreacting and are wanting to control other people in the process. In this case, that's your own husband who is probably really being hurt by the entire situation.