For old family heirloom jewels, it's actually a good thing that Tiffany's didn't want to set it. Here's why:
Older diamonds and gems were cut differently back in the day. Usually by hand, and that meant that the people who were cutting them had to know what they were doing. That means that the older gems you have are worth much more, even if cut with a modern cut, than what you see on the market.
When you go to a chain jewelry store, you are actually giving up certain protections because you are dealing with a corporation. That means that if along the chain of people that get your diamonds into the hands of a jeweler there happens to be a dishonest person who swipes your gem for a fake, you end up having to sue the company and deal with their army of lawyers, having more to lose than they will.
Start with the actual pieces you were left. Look for inscriptions, engravings, or any kind of paperwork, receipts that mention a jeweler. You want to go there first if they are still around. Next, call the GIA and EGL, they will have listings of certified jewelers in your area. If you know professionals who own fine jewelry, ask for their recommendation also. Start with five jewelers, then visit them, interview them. Ask them what they can do and what they can't. Ask to see a portfolio of their work and if their work is ensured. Ideally you want to find a jeweler that owns their own business and is discrete and quiet, who doesn't need to advertise to get business because their customers always recommend them to their friends and who will accept your commission because it's a chance for them to work with much larger stones.
And I wouldn't talk about your super expensive jewelry online. I would also say never go out of the house alone without at least your significant other in tow when you have one of these pieces on your person. None of us want some of stupid stuff that happened to Kim Kardashian to happen to you, and a little caution never hurts.
The right jeweler is out there. You just have to find them.