Being to Suspicious or Right to be concerned?

[Guest]

My b/f and i were at his apartment last night. (Together for a year) We were in his bedroom and his phone rang indicating a text message. It was almost 2 am. He goes to the other room to get his phone and said it was his sister. He stays there for a minute or two and i guess he called her. He comes to the bedroom by the window to get better reception to speak. His apartment does not give good reception. The girl is talking about a guy and then he said " Did you tell your mom?" If it was his sister, why would he refer to their mom as her mom. It's not his step sister or anything. I said that to him when he was done talking. He said his sis was having a guy a problem(she is older than him--i think 35 or something)Then he said:LOOK--the text is from her and who he spoke to was her. He asked "i never did that" and obviously my answer was no. He probably knew what i was thinking when he said look who it says its from--if a person is sneaky enough, they can easily just change the name to someone else. He said call the number then. I didn't call. Does it sound weird that he said your mom when it is his mom as well or am i being too suspicious and just overanalyzing. Was he just hoping or figured i wouldnt call or should i trust it was his sister. I agree you need to trust in a relationship but i also do not want to be naive and just completely trust in every single situation. I am tempted to copy down the number and see if it is her. Would anyone else have noticed this when he said it? I couldn't help but say something to him and now having a hard time deciding to let it go or not. I have not brought it up since. The thing is--if it was another girl and she is talking about a guy problem--that means they have developed some sort of closeness and he tells me he rarely even talks to other woman. Is it weird for a girl to text a guy at 2am about a guy problem that wasn't that serious--something somewhat funny? Meaning--it was more than likely his sister?