Effective ∑-n and ∑-∑- interactions are derived from the G-matrix calculations for {n+∑-} matter and employed in the investigation of hyperon mixing in neutron star matter. The threshold densities ρt(Y) at which hyperons start to appear are between 2ρ0 and 3ρ0 (where ρ0 is the normal nuclear density) for both Λ and ∑-, and their fractions increase rapidly with baryon density, reaching 10% already for ρ ≃ ρt + ρ0. The mechanism of hyperon mixing and single-particle properties, such as the effective mass and the potential depth, are analyzed taking into account the roles of Y N and N N interactions. The resulting equation of state is found to be too soft to sustain the observed neutron star mass Mobs = 1.44M⊙. We discuss the reason for this and stress the necessity of the "extra repulsion" for Y N and Y Y interactions to resolve this crucial problem. It is remarked that ρt (Y) would be as large as 4ρ0 for neutron stars compatible with Mobs. A comment is given regarding the effects on the Y-mixing problem from a less attractive Λ Λ interaction, newly suggested by the NAGARA event.