simp_rw [← (fun i ↦ M.er_cl_eq (Xs i)), ← (fun i ↦ (hIs i).cl_eq_cl), M.cl_iUnion_cl_eq_cl_iUnion,
  er_cl_eq, (fun i ↦ (hIs i).indep.er)] at hsum
ι : Type u_1
α : Type u_2
M : Matroid α
B I J X X' Y Y' F : Set α
Xs Ys Js Is✝ : ι → Set α
i j : ι
e f : α
inst✝ : Fintype ι
hXs : ∀ (i : ι), M.rFin (Xs i)
hXE : ∀ (i : ι), Xs i ⊆ M.E
hsum : ∑ i : ι, M.er (Xs i) = M.er (M.cl (⋃ i, M.cl (Xs i)))
Is : ι → Set α
hIs : ∀ (i : ι), M.Basis (Is i) (Xs i)
⊢ M.SkewFamily Xs
ι : Type u_1
α : Type u_2
M : Matroid α
B I J X X' Y Y' F : Set α
Xs Ys Js Is✝ : ι → Set α
i j : ι
e f : α
inst✝ : Fintype ι
hXs : ∀ (i : ι), M.rFin (Xs i)
hXE : ∀ (i : ι), Xs i ⊆ M.E
Is : ι → Set α
hIs : ∀ (i : ι), M.Basis (Is i) (Xs i)
hsum : ∑ x : ι, (Is x).encard = M.er (⋃ i, Is i)
⊢ M.SkewFamily Xs