I feel the same as Mandi.
Tbh your brother shouldn't have even felt the need to ask for something that belongs "in family". If the step-brother had taken care of your Grandfather or was his bestest friend - then maybe, otherwise they should butt their greedy noses out of business that does not apply to them.
I know the cross necklace my Grandmother had set aside for me is my most precious possession I own - its not worth crap but the sentimental value is astronomical.
Also your Step-Mommy sounds like a real - rhymes with witch.