Story About Our Younger Bull

My gf (Alana) and I are both 31 and we live together in a condo.
My gf has a younger brother who is 25, all his friends are about that same age except for Steve who is 22. A while ago my gf and I were out drinking and Steve messaged her over fb about something totally innocent (can't even remember what), the timing was perfect because my gf was drunk and horny and I guess always found Steve’s arrogant attitude hot despite never admitting it prior. They end up flirting and nothing really comes of it but if certainly opened a door. In the next weeks the relationship becomes more and more sexual and she finally tells him about our "arrangement". He finds it super funny and his directness shoots into overdrive. The following Saturday at around 11pm he messages her. His great timing, I suspect, is no coincidence since she's out with me drinking at the time. He tells her to come over, she plays it off like he's not serious, then he sends her a pic of his cock (which he's never done before). This kid had a pretty serious cock and my gf's entire demeanour changed. She ends up going over and they fuck. She's texting me sporadically and ends up home around 3am.

Just for reference to compare us:

Me: 31, 5’11, 175lbs, hit the gym 3-4x a week, 5 inch dick on the thinner side.

Stever: 22, 5’9, skinny, think skateboarder type, I don’t know how big his dick actually is but it’s thick af and considerably longer than mine.

Alana: 5’2, italian, great ass (really like perky as hell), very innocent looking and looks much younger than she is, D cup tits.

Since then he keeps pushing the envelope, and fast. The weekend after that was quiet and we stayed in. We are in bed around 1130 when she gets a text from him. It seems like he's been out drinking and he wants to come over. I can tell Alana wants it despite acting apprehensive. I also find how forward and aggressive he's being to be hot so I agree. But then he tells her to "get rid of me first". I am in my Pjs and half asleep. Where am I supposed to go? So I tell her no I'm not going. She kinda pleads and says it won’t take long and asks if I could just wait in the bathroom. I agree. Steve says that’s fine. So when I hear the knock at the door I get out of bed and head into the only bathroom (adjacent to our bedroom). I can’t hear much until he starts railing her and all I can hear are sounds out of her I’ve never heard before. He must have fucked her for an hour straight. Then I hear loud gagging, like a lot of gagging, and loud, for a while, then him loudly moaning, followed by some chatter and then him leaving. Alana walks into the bathroom covered in spit and cum with a kind of a half smile on her face. I say “kinda a half smile” because the amount of cum and spit on her face made it legit hard to even make out her expression. She cleaned up and we went to bed and she thanked me and gave me a kiss and a swear I could still smell his cock on her breath.