Best thing is to research the law - check with organizations like your NRA - and decide in advance what you want to do with it, depending on what it is and the applicable law.
If it's an original percussion, a non-firing replica, a conventional air pistol, a deactivated cartridge conversion, or a deactivated modern percussion, you can own it without any legal hassle.
If it's a firing condition cartridge conversion, you can get it deactivated but you can't get a fAC for it.
If it's a firing condition modern percussion, you can get it deactivated. Or get it on FAC: but you'll need to join a pistol club, and shoot it there regularly.
A Brocock TAC? I know that when they were banned, it was a legal compromise: present owners could keep them but could never transfer them to anyone else. Therefore, I don't think you could legally keep it if it's a firing condition TAC. But I would guess that you could, if you get it deactivated.
In any case, it's a beautiful pistol, has a family history to it, and obviously means something to you: so I hope that you will find a way to legally keep it.
Jim