As a fellow Nova Scotian, I can see where the connection to Boston/MA is coming from, the two places are in very good terms, Boston being one of the main sources of aid after the Halifax explosion. (We actually send them a big ass x mas tree every year) and various other connections
While I could see Nova Scotia being a large trade port for a lot of the Atlantic coast, and a good connection to Boston. I wouldn't be surprised if there were some nods or parts of lore that factor in this relationship. I Don't see NS just up and joining them. Pre great war Canada took the annexation pretty hard, and there would likely be a lot bigger things on their minds than joining with one particular commonwealth.
Like mentioned earlier, This area was just 1 of many commonwealths, and it happened to get that as a nickname.
After the great war the Commonwealth is just as chaotic and uncivilzed as DC was. Wouldn't be anyone to give NS orders.
My interpretation is that NS and the Maritimes would at the very least get hit with crippling amounts of radiation, which would be more than enough to send it into anarchy after the nuclear exchange, and individual cities, factions, and militias would rise out of that. Sure the provinces would keep their names (Canada is said in the lore to have retained it's provincial municipalities after annexation due to its size and low population density) but it would largely just be who had the bigger stick.
I could see at least some parts of NS forming trade routes, fishing, etc over the Atlantic with a lot of other places. with NS being so far out of the way its unlikely they suffered any direct hits, so ships, and architecture would be in much better shape than places farther south.
As someone who's lived in Nova Scotia all my life, it's awesome to see Fallout 4 being set in Boston, the kinds of terrain you see are somewhat similar. Probably as close to NS a fallout setting will ever Be. I hope they have some rural, back-woodsy areas in the Fallout 4 map, as I'd love to build a house similar to my own. (Live out in the middle of nowheres. Halifax is way too busy for my tastes )