What isolation? The children just become fluent signing with their parents, just as any child will spontaneously pick up their parents' language.
Yes, there would be a level of isolation. It needn't be extreme, but I can't imagine there would be none
I've known plenty of people and families like that, it was never an issue.
Maybe more connected? If you are using a language that few others know, seems like it would enhance the "specialness"