Yuuenchi

Main character's mother remarried three months ago bringing new sister Rika to the family. Rika has male phobia and she can only talk to protagonist using kansai dialect. Rika starts to have strange phone calls and gets signs of being followed by stalker. One night she gets stuck at the amusement park. Protagonist is in the same place due to meeting with his student pals. Can protagonist prevent his sister's rape?

[From vndbreview]