AITA for suing my brother over a family heirloom he gave to his fiancée?

I come from a family where heirlooms mean a lot. Our grandmother left us an antique diamond necklace that’s been passed down for generations to the first daughter in the family. Since I’m the only daughter of this generation, it was supposed to come to me.

My brother claimed grandma told him in private that it should go to him instead because he’s “the most responsible.” I didn’t want to cause drama, so I let it go, even though it felt unfair.

Last week, I saw on social media that my brother gave the necklace to his fiancée as an engagement gift. She posted a picture wearing it with the caption, “Feeling like royalty with my new family heirloom.”

I confronted my brother and reminded him the necklace was meant to stay in the family. He said, “She is family now. Don’t be petty.” When I asked for it back, he refused, saying it would ruin their engagement.

I decided to take legal action to get the necklace back. Now my brother is furious and calling me selfish. My parents think I’m overreacting, but some extended family members are on my side, saying he never had the right to give it away. His fiancée even messaged me, calling me a jealous drama queen and telling me to find my own man to buy me jewelry.

The whole thing has caused a family feud, and now my brother and his fiancée are threatening to uninvite me from the wedding.

AITA for taking this to court over a necklace that was supposed to be mine?