Nothing will prevent sinew or wood from changing M/C as the R/H changes. Both are hygroscopic and that means they take on and release moisture to the atmosphere as the R/H changes.
My Massey Medicine Bow(sinew backed hickory) has changed weight from 45# to 65# depending of the R/H at the time. That bow was sealed with Massey finish(2ton epoxy thinned with acetone), 5 coats of Tru-Oil and a spray of satin poly.