Motivated by recent experimental observation of the quantum spin Hall effect
in monolayer germanene, we study the topological phases of twisted bilayer
Kane-Mele model with time-reversal symmetry and spin sz​ conservation. For
large twist angles the helical edge states from the two layers are unstable and
the system is a trivial insulator. At small twist angles however, the emergent
moir\'e flatbands can be topologically nontrivial due to inversion symmetry
breaking from coupling to substrate. Each of these flatbands for each spin
projection admits a lowest-Landau-level description in the chiral limit and at
magic twist angle. This allows for the construction of a many-body Laughlin
state with time-reversal symmetry which can be stabilized by a short-range
pseudopotential, and therefore serves as an ideal platform for realizing the
so-far elusive fractional quantum spin Hall effect with emergent spin-1/2 U(1)
symmetry