A system for treating disease involving branching vessels of a mammal system can include a main graft assembly (i) having a lumen permitting fluid flow therethrough, and (ii) configured to expand within a first vessel of a mammal； and a branch graft assembly including a branch cover (i) having a cover lumen permitting fluid flow therethrough； and (ii) configured to expand within a branch vessel that branches from the first vessel. The branch graft assembly may also include an expandable branch stent extending within the cover lumen. The branch graft assembly may further include a branch sheath (i) extending between the branch stent and the cover lumen, and (ii) constraining radial expansion of the branch stent within the cover lumen.
