WonnayWombat
my two stepdaughters are 16 and 15, and they have always been fine with each new baby. I married their dad when they were almost 5 and 6, and I must admit that with my first baby with DH, they were a bit jealous, but probably no more so than most other children with a new sibling.
However, this time the news has not gone down as well, mainly because we are a bit cramped for space, need to move, and both of them have important years at school...at least, that's what I think is the issue. I'm sure they'll be fine in time, as always!
With your SC being older, the news may be a bit disconcerting to them at first,because they're older and will have to deal with having a much younger sibling, but, if it seems that way, all you can do is to assure them that there is enough love to go around for their needs at the stage they're all at.No one will be forgotten or sidelined.I doubt they'll be particularly put out by it to the extent that it would cause problems (they could even be excited!), so just break it to them when everyone is relaxed, maybe during a meal or something?