Gf hooked up w my brother

So this is the first time I’ve posted in here so bear with me if I’m not entirely correct. I (22m) work as a GC (general contractor) and travel out of state a lot for work. I make pretty good money because of the work I do but it takes me away from home a lot. I live with my gf of 2 years (20f) and she would stay at home most times I was on work trips. My older brother (27m) on the other hand, is struggling to say the least. He works as a warehouse supervisor and wasn’t making the best money. Because of that, he was losing his apartment that he was staying in and needed a place to stay. Me being the big hearted person I am. Told him to come stay at my place free of charge till he got back on his feet. He had been living with us for about 6 months. And surprisingly I spent alot of that time at home. But recently I went on a month trip to AZ and had no concerns about them being there together. But when I returned, I found messages on our iPad of her and him talking about how much they love eachother and them sleeping together. It was probably the biggest slap in the face of my life and I’m still heartbroken over losing a part of my family, and the person I loved for the last 2 years. I’m really kinda lost rn and need just SOME words of advice because I’m not sure how to handle this alone. I don’t really talk to anyone other than my best friend and her.