There have been ancient darts found in ice fields with the sinew intact.
paste the link and remove the spaces in www
https://ww w.cbc.ca/news/canada/north/yukon-atlatl-ice-patches-1.4809947
keep in mind that backing a bow with rawhide to help prevent a tension failure on the back has merit. the bow is essentially designed as a wood bow, and the rawhide, while it works some, is not considered a major working element of the design. It could be regarded as "insurance"., and the design length needs to be appropriate for the desired draw length.
A sinew back bow typically has substantially more sinew, such that the sinew provides all that is needed to let the back bend, and then some. these designs can be shorter for their drawlength.
you could use sinew for backing or insurance in place of rawhide, but it might be better saved for a bow that was designed to utilize it better.
Branches have a different kind of wood than trunks, orientation matters, as the topside of the branch is different from the bottomside.
Hope someone who has built a bow from juniper branchwood can comment more about your design.
Will it bend through the handle?