Dealing with family can be a very tricky situation which can lead to hurt feelings. Money and family tend not to mix well. There is no amount of silver that is worth sacrificing family realtionships IMO. For this exact reason, I absolutely try to avoid situations where friends/family and money intertwine. It rarely works out well.
My advice would be to be 100% honest and go from there. If it was my sis-in-law, I would tell her exactly what the melt value was. I would find out EXACTLY what the local LCS/pawn shops were paying and share that as well as what she could expect to receive if she sold on ebay (minus fees) or to one of the big metal buyers such as Provident or APMEX. Add in that you'd like to buy the coins, but you can't pay "full" price as there is no room for you to make any money. Then let her decide what she wants to do. She may want top dollar. If she does, I'd either pony up the cash or let them go. She might choose to give you a "family discount." If so, great. If not, don't get upset over it. Would you sell her your silver at a "family" discounted price? ~$400 worth of silver isn't worth a hassle. Expecially not a family hassle. No amount of silver is worth that.