@NakedBanana
I'd hate it OP, I can't tell you why, as I don't know!
Yes I'd be worried about inheritance, but as you say that has nothing to do with you. Also the long term, what if they split up? Would your child still have contact with with her new step sister.
My mind would be spinning too but it's your child he needs to be discussing it with.
Also it's all a bit strange, why the need to adopt?
Yeah complete mind fuck. But don't worry the mumsnet keyboard warriors will be along soon telling you what a terrible person you are!
I'd feel exactly the same. I just can't see any actual need for it. Unless they are intending to have more children together, and do not want the stepchild to feel in any way disadvantaged. In which case your son's inheritance is going to be vastly diminished anyway.
I have a friend who adopted his partner's two children then went on to have his own with her, but
a) they were very young and had no relationship or contact with their natural father, and
b) he didn't have any existing older children.
His parents disinherited him and diverted his share of inheritance directly to their 2 grandchildren that were his biological children to avoid any of it going to their step-grandchildren or their mother. (long backstory.)
This friend did eventually break up with his wife and although he's still close to his adopted kids they are adults so day to day finances and maintenance don't really come into things at this stage.
Adopting or not, I think it's very hard on children who are presented with a new 'parent' (particularly a new 'dad' as he is most likely to be living in the child's main home as a permanent fixture) only to have that relationship break down. They then have to navigate this weird period of pretence where the stepfather who they've been encouraged to see as 'dad' weighs up whether he really wants to be 'dad' and continue paying to support this child or not, now that's he's not with the mother. And most often the answer is No, he doesn't. After a half hearted attempt at keeping the parent/child relationship going it pretty soon peters out. The child came as part of a package deal that he is no longer interested in.
For some kids this happens not once, but several times and if they don't have a relationship with their bio dad this can be particularly hard.