Yes, my eldest were 9 and 11 when we had our 3rd - not remarriage or anything, just... life.
Obviously your context of a child with SEN needs to be taken into account, but you might find it does that child a lot of good to be able to take responsibility and to be looked up to by the baby.
My own experience has been wonderful: the two older ones were big enough not to be at all jealous, and small enough not to be embarrassed by parents still reproducing at that age! Our eldest in particular has been like a fun, cool uncle to his little brother, and has helped him at school in ways that we couldn't really.
I also found I had time to enjoy his babyhood in ways I couldn't with the other two: I was too inexperienced with the first to feel entirely confident, and too flipping busy with the second because the eldest was still a toddler. Third one was just so much more relaxed, for me and for him. Plus I could leave him with the two big ones if I needed to nip to the shop, where I often had to drag the other two out with me, as obviously leaving them alone wasn't an option!
The only downside is that when the two big ones went off to university he was a little bit on his own - but we got a dog, and that's been great (we've always had cats, but this is his dog, which is very different.
We also started inviting a friend on holiday with him - the sort of thing people often do with only children, so I don't think he lost out too much. Possibly an even better solution would have been a fourth child shortly before/after the 3rd, but again, that didn't happen.
So yes, unless you have real organisational issues with your SEN child, I'd absolutely say go for it. I don't regret it for an instant.