Woman Wont give me my dead moms necklace

LONG STORY My mom passed away January, 28th, of 2017. About 5-6 months later my dad got another Girlfriend after my parents were married 20 years. Never was rude to the woman tried being nice to her. I moved out of my dad's house and didn't see my dad for about four months (not on purpose, we just were always busy) I finally got free time on my hands & I rode up to see my dad & I never knew they had made plans that day to go do something. When found out I was coming to see him he called her & told her he wasn't going to be able to make it because he hadn't seen me or my husband in a while & he wanted to see me. She flips out & they end up getting into a fuss & breaking up for a week and then got back together. (About two years before my mama died she bought my dad a Harley Davidson necklace)My dad says he goes to stay the night at her house and before he went to sleep he laid the necklace on the night stand and went to sleep. He said he woke up and she was wearing them. He said he ask her for them back but she ask to wear them for the day seeing as how they were supposed to spend the day together My dad let her wear them for a week and then she broke up with him blocked his number, his FB, etc. Well eventually the necklace was supposed to be passed down to my 13 yr old brother, so my brother messaged her on FB asking for it back because my dad has no way of getting in touch with her and this is what she said..

Whatever happens between my dad & her is between them. Those necklaces belong to my 13 yr old brother that OUR mom bought not her. I'm so furious & yes I realize the date of the messages but they were just now shown to me.