R These Signs of Cheating?

[BetteDavisEyes]

My brother has been in a relationship with a girl for the past 3 years. Lately, he's been extremely secretive & has been shying away from family functions. When he does come, she rarely attends anymore. My brother has been fighting with my mom b/c she's nosy as hell & can't mind her own business over the state of their relationship.
The other day, she left to work while he dropped off her daughter at school. Two of my sisters went to her work for some coffee & she was not there. They were told that she was off that day but she had told my brother she had to work.
I know it's flimsy but I am starting to worry. Maybe it's too early & I know it's definitely not my business but I have always liked her & her daughter. This would definitely be major blow if she's stepping out on him. I don't want to overstep my bounds which is why I'm going to let them handle their own affairs but I'm confused.
Also, she has threatened him repeatedly that if he doesn't marry her after a while, she's leaving out of state.