Man'in Chijo Densha ~ Hosoi Yubi de Shigokanaide

Main character is a normal, slightly short boy. One day he is late for school and in a hurry jumps into a female-only train car. Two women attack him there and make him ejaculate. Next day same two women sandwich him again, and this time they allow to insert. Crying boy tells it all to his beloved sister, and sister promises to find and the abuser and take revenge.

[From vndbreview]