I think my husband secretly gifted my sister a necklace...help!r

Mamas…I need positive thoughts, vibes and prayers…whatever you do…I need them…before Christmas I found a necklace tucked away in my husbands truck with a bow on it…I thought I stumbled across my Christmas present from him so I didn’t say anything and ruin the surprise…it was beautiful and I was over the moon…Christmas comes around and nothing…I was let down and thought maybe he forgot about it…evening rolls around and we head to my moms house for Christmas dinner and I notice the same necklace on my sisters neck and the box is no longer in his car…I have been stewing on this for days maybe thinking she just had the same necklace…but they do kind of flirt and joke around a lot so at the same time I am now thinking they are seeing each other behind my back and I cannot shake the feeling…how do I go about approaching this? If I am wrong and accuse then I ruin relationships…idk what to do… please help…I am heartbroken