Stepsister's Wet and Wild Struggle

My stepsis, Trinity, has a problem with her boyfriend, and I might be the ONLY person who can help her solve it! She finds me in the living room, and sits down for a chat.  My stepsis tells me that her boyfriend has been stopping in the middle of sex and she doesn't know what the issue is since he won't tell her anything.  So, she wants me to help her figure out if there is something wrong with her vagina.  Obviously, as her older stepbrother, it IS my job to help my little stepsis work through her problems.  She shimmies out of her jean shorts to reveal a cute pair of yellow underwear that she pulls off before laying down in front of me and spreading her legs.  I can't see any problems with her tight pussy, and so I dip in a finger to feel if there is anything abnormal happening inside her.  My stepsis's pussy is HOT, WET, and really really TIGHT, which gives me an instant hard on...