I assume the split is on the belly?
Did the sinew pull the bow into reflex?
Sinew can actually pull enough to crack bellies. But, more likely, the crack was already there and the sinew pulling just opened it up enough that you can now see it. If it runs along the limb and not off the side, as opposed to across it, you will be fine.
Sinew does nothing on the belly if that's what you mean by "adding another layer". Sinew is tension (stretch) strong but pretty much worthless in compression (like you get on the belly).