Do I have a right to be mad at my brother?

I'm sorry this is kinda long but here it goes:

My brother and his girlfriend have been together for almost a year now. He goes through girlfriend like he goes through underwear so she is the longest relationship he's had. He's usually a huge player/cheater and goes for sketchy girls but this one is different. His girlfriend is super sweet and I really like her. Anyway, He got back from bootcamp/AIT September 18th. His girlfriend went to bootcamp August 9th and she's currently still there. Today I was on the phone with our older sister and she told me that our brother had brought a girl and one his friends to her house the other day to hang out. Later she texted him and asked what was up with the girl. He told her that they were talking. My sister asked if he was talking as in to date her and he said maybe. She asked about his current girlfriend and he told our sister that they broke up. I checked their social media accounts and it still says that they're still dating on Facebook and Instagram. Also, his girlfriend called my mom yesterday because she had free time and was given a few minutes to just talk. (His girlfriend has gotten really close to our family.) My brother and his girlfriend had plans on buying an apartment together after she got out of bootcamp and my mom talked to him about it today and he told her that he needs to fix his truck first before he starts savings for an apartment.

My point is, we don't really know if they're broken up or not. He maybe telling us the truth but it's unlikely. My mom, sister, and I all think he's cheating on his girlfriend. Ik it's not my relationship so technically not my business but it just makes me so angry. This isn't the first time he's cheated on a girl and I don't understand why he's like that. So, do I have a right to be mad at him?