Why should me friend be annoyed about me and her brother?

My apologies first of all for answering this some four months late, lol… I hope what I have to say is still of some use to you, perhaps for future appliction.

As it seems you are already aware; this kind of thing is quite common. Many people are feel uncomfortable when their friends take interest in one another or when friends take interest in relatives/ close family. The reasons for this vary. Sometimes they fear that neglect will kick in (that is that the two of you will pay attention to each other and not to her), sometimes they feel jealous (that two people they know are paired up whilst they are not), etc.

I think a good long talk is the best remedy for such a problem. There is nothing to be ashamed of for either of you, so being open should not be so difficult. You need to tell her that you and her brother just want to try this out and see how things go. At this point it is nothing ‘serious’, but the two of you like each other and were she in your position, you would not stand in her way.

Promise her that you will not make the same mistake that many others do, in that you will not allow yourself to detach from her and that you’ll still be her friend and (despite the fact you are not close friends) you will always be there for her as has always been the case. Just reinforce that you will allow nothing to change between the two of you unless she initiates that change.

If she isn’t completely comfortable, that’s okay. Very rarely is anybody going to jump for joy when they hear their brother is going out with a friend. But you just need to put things into perspective for her. Often the thought of such a scenario is more difficult to comprehend/ ascertain than the reality.

Give it a shot and if you see that there is friction between you and her brother, or you and her, then maybe stop and re-evaluate things then. For now, go with your heart.