The biggest obstacle to having anyone steal your idea is sheer laziness. For cofounders, it'll be worse than that – they already know roughly how much work is involved, and already have a team of dedicated souls to execute it. The worst possible mistake at this point would be to try and replicate all that on their own, or with a new team (who would, perhaps, also know on some level that they're working with/for someone willing to steal another's ideas, bad scene…)
Practically speaking, it's much more likely that your team will hang together through product launch, and then have a falling out over a funding round issue, exit offer, or disagreement over how long to hang in without revenue before trying a pivot.
In the meantime, get your nose back on the grindstone, build whatever it is, and launch the damn thing.
At the idea to prototype stage of product development, how can you make sure that none of your co-founders run off with your idea and implement it alone?