At the end of the day, most people i know give their kids nicknames which are either shortened versions of their actual name, or cutesy names (one friend calls her daughter pickle, for example).
As such the actual name will only be used on official stuff. I had 3 good friends in school, 2 named catherine, one Kathryn. all pronounced the same. One was Cat, one Cath, one Katie.
Even if both of this little boys half sisters end up with the same first and middle names, it is very, very unlikely both will go by the same nickname or get confused with each other, especially as the only family they will have in common is him. Most people will only know of one of his sisters so not get confused between the two.
Yes, it is possible it is a name the OP's husband liked and discussed with his ex as well as op, it may be ex didn't have a name chosen and just loved the name op mentioned, pure coincidence ex had always loved the name, or it could be a spiteful act trying to name steal.
Use the name OP, anyone who knows you will know you've named your bump that from day one knowing it was a little girl, and anyone who didn't know and thinks you stole etc, obviously doesn't know you or isn't important in your life anyway. Does it really matter if your ex's family/friends think you used the same name, you're unlikely to ever meet them, and if you did, you could put them straight you had that name picked from the beginning and it just happened both of you wanted the same name. Not that you'll ever need to explain yourself to anyone.