Feelings are complicated matters... right?

I can't state for a fact that you maybe in love with him. As you have said it is a brotherly love. I might be stating the obvious here, but as you have said he spends more time with his girlfriend more than with you. I really do feel that your feelings are platonic wanting to protect your friend like any sibling will do to a brother or a sister.

I think your being develop into that guy and not deny the fact that you like that guy but hide that feelings because you still consider him as your best friend. Why! Not tell to him that you like him instead to be jealous...

When it comes to a relationship, sometimes your mind is in one place and, your emotions are some where else. There are different flavors of love. You might say that he's like a brother but you truly have feelings for him if you experience jealousy. Some people are naturally protective WITHOUT feeling jealous. I think you have a very special place in your heart for him and maybe it's fear that you could lose him to someone else.

Even real life sisters get jealous of their brothers' girlfriend! I think it is but a natural feeling especially when it's just been the two of you for quite some time. And now another person comes in the picture and suddenly it's no longer you and him, but he and his girl. I'm sure you will get over it soon; maybe you two have been so close and always do everything together that's why you feel like you're being left behind. Who knows maybe you can even be friends with his girl, and instead of one guy best friend, you'll have two-him and his gf.