Tell me about my girlfriend?

hi i am 27 years old and my girlfriend is 21 years old ,once my younger brother who is 22 misbehaved with her ,she told this incidence to me...and when i asked whom did u told except me ,she told her sister,but after some days she told she only informed me about this????i asked her did u talk to him anymore now,she told no.....once my friend told me that once a guy was misbehaving with her she told my younger brother about this......i generally remain outside due to my studies????can i trust her???once i told her that i came to know u and my brother had affair earler then she told no and told if u dont believe ask your brother.if she dont like him then y did she take his help?