Did your girlfriend’s brother-in-law have a child marriage? You say they loved each other during childhood!! What is the difference in their ages?
It is quite obvious that you are getting hyper about a non issue. It shows lack of stability in your relationship. Either your girlfriend wants to make you feel jealous by talking about her brother-in-law or she is giving you signals that she wants to break up with you.
A few photographs together and exchanging of mobile numbers is not a big issue. Remember, they are related and it is natural that at a wedding there will be instances when they bump into each other. However, if you have proof that he sent her a message that he wants to take her virginity (have sex with her) then you should confront him and even speak to your girl friend’s sister and show her the proof you have.
But for all this to happen, you need to understand the stability of your relationship and know how far your girlfriend will stand by your side. You have spoken about your frequent quarrels, but have not told us about the issues involved. If it is about your jealousy and doubting her character, then she has grounds to be annoyed, since she has already told you the truth and her response.
You need to have a long, hard look at your relationship together.