How long have you been married? How long has it been sinse you were actually in a relationship with the Jewelry boyfriend? It honestly sounds like you still have feelings for this boyfriend. Those of us that are married, we do not associate this much feeling to anything to to do with old boyfriends.
Sure, we may hear a song and recall a time or an experience, but we quickly remember the good reason we are with our spouses. We may come across a trinket and recall all that went along with receiving it etc. But honestly, I have either given away those things, thrown them away, or I have some odds and ends packed away with other childhood memories and when I come across them, I laugh that I still have those things and lesson learned! I actually have shown our daughter those things and she just shook her head. My mom even has some old notes and trinkets from WAY, WAY back in the day. I love to look at them and ask her to tell me the funny stories about those "old guys".,
"I would never sell it."
" have only worn it a few times while married to my current husband and feel weird when I do."
" I was thinking of mailing the necklace to his sister"
3 questions for you. Why? Why? Why?
Why not sell it? You do not feel like you can wear it.
If you honestly feel weird about wearing it? Then you need to figure out why you still have ANY feelings about him and you need to decide how to get past that.
Why on earth give it to his sister? To be honest, that sounds like you are trying to somehow have contact with him by using his sister.
Sell the piece of Jewelry and then either donate the money or consider if you may want speak with a professional about what is REALLY going on here.