You are not over reacting . My husband starts questioning me if I am ever not at my seat at work and don't attend his phone or if I dont attent his call on mobile. If my husband was at your place he would be very concerned.
She was very quick at asking for an apology I am surprised ! most of the girls would never use the words "maaf kardein " I would die but never say these words to my husband. And NO its not common for girls to talk like that.
There is no such thing as soo called brother. I won't suggest you to jump to any conclusion. Be concerned , try to dig info about her activities , its your right you are all sincere to marry her. If she was at your place I would suggest her to do the same. Marriage is a life time commitment be very sure before you get married.
Hope for the best but prepare yourself for the worst !