Nope, flax is the raw material linen is made from (from the flax plant Linum usitatissimum).
Sisal is from a species of agave (Agave sisalana, surprise surprise).
They have rather different properties. As far as my research data and experience go, flax is extremely strong, and stacks enormously beyond 1% stretch.
If your flax backing is too thick, it will force the underlying wood entirely into compression. When you then draw the bow, the tiniest flaw in the belly (a hint of a hinge) may cause such a crushing of the wood that a true hinge quickly forms and is immediately exacerbated in a chain reaction leading to a tension failure of the flax. I've experimented with flax backings and had explosive bow breaks because of that.
Sisal is much cruder (thicker fibers), but stretches more easily and doesn't stack like flax. It can stretch at least 2% and has a much lower modulus of elasticity than flax (it is more elastic, requiring less force to stretch), more like strong wood species. Basically, it behaves like a stretchy bamboo backing, but one which you can tiller also on its back: when you applied too much of it, you can sand it down (moderately!). When you put too little on it, you can always add a bit.
