If Princess Elisabeth marries a foreign Prince, will he still not be considered a Prince because he was born a Prince?
I’m not sure I follow.
Let’s ignore their current relationships and say she married either Nikolai or Felix. Regardless of what Philippe did or didn’t do with a Belgian title Nikolai/Felix would still be HH Prince Nikolai/Felix of Denmark. So at the very least we have HRH The Duchess of Brabant and HH Prince Nikolai/Felix.
My guess is that regardless of who she married, Philippe would make Elisabeth’s husband HRH Prince (name) of Belgium as it would be very strange for the husband of the crown princess to not have a title.
As for the spouses of her siblings…we’ll time will tell but seeing that their spouse and children will all be HRH Prince/Princess of Belgium I expect will see them be as well.