What glue are you using? Sintra being a plastic, any sort of fresh cyanoacrylate glue should bond it so well that you'll pretty much be unable to separate it without cutting out the joined area.
Additionally, it being a thermoplastic, you can heat-form the panels into the curves they need to be if you'll not already doing that. Makes assembling the whole thing much easier than trying to stress flex the sintra. Have good ventilation though or wear a respirator, as it is PVC and does release fumes.
Using super glue, epoxy, and bondo. The dome is super strong but the lower part I feel will crack if dropped. I know some people use fiber glass inside, just wondering if there's other and easier ways to reinforce the helmet.
So it's already an assembled build and you want to reinforce it then?
If it's a work helmet, slushcast the inside with some thinned bondo (google 'rondo helmet')
If it's a wear helmet, then fiberglass is probably your best choice if you want to not kill your neck. Paint it over to protect from any possible fumes though.
But as Fett 4 Real said, all of these are still going to be brittle though, it's just the nature of these polyester resin products; it's why these are usually work helmets and get recast into urethene. You can order some specialty products that are less brittle, but they're going to be expensive. I'd just try to keep as much natural Sintra as possible, because at least it's light and flexible. Maybe just spam extra thin CA glue at all the joints and try not to drop it. Or hot glue.