Welcome to hitched. I'm sorry to hear of your issues with family & sympathise. I thought my parent-in-law problems were bad until I read about yours! There are always 2 sides to a story, so in fairness, I tried to consider their side also.
From their prospective, is there any way they suspect that their son wanting to see his birth family was spurned on by you? Thats the only reason I can see for them being so nasty, & even so, absolutely no excuse at all. Maybe they are feeling that their son may reject them, not only with getting married, but also finding his birth family & possibly having more to do with the new family in the future.
They must have thought of this for many years, infact since adopting him. I wonder if he could suggest an organisation for adoptive parents to help THEM come to terms with THEIR insecurities?
Legally, they are not only nasty but abusive. Abuse can be verbal, its not always physical. I'd have got a restaining order long before this! All the stress is detrimental to your health, as shown by being on anti-depressants! If you have children, would you be happy for them to be subject to such abuse?
I personally, couldnt stand to see them again, even the mother & sister sucking up. I wouldnt invite them to the engagement or the wedding. There are only so many chances you can give people, & I think you have been more than accomodating. Maybe there are other issues going on you arent aware of & some miracle may occur & they appologise & change dramatically.
On the highly likely chance that this doesnt happen, I'd be organising an intimate wedding abroad with your chosen family & friends. Good luck.