Thank you, Omion, for the added information.
I suppose I am playing “mother” here. I think knowing you are so young, I just hate to see you going through this. It is not that he doesn’t deserve someone to not bail on him when he is going through this time in his life because he does.
I just wouldn’t want you to stay in such an emotionally charged situation unless you are completely dedicated to his happiness and this relationship.
Your questions towards the end of your post is what got me thinking along the line of you not being completely invested in the relationship. And that is perfectly ok of course! You are very young and he is older which again is ok but there is a little difference in what is going on in the dynamics.
You are probably looking for a relationship where it is about both of you and “the relationship” and now the relationship is taking on another life. This life is going to be about his brother as it should be.
Your question is are you OK with that.

If it were me, and I had 20/20 hindsight, 🙂 I would let the relationship move along without any interference from you. Just see where it goes. He might realize that he is not putting you in your rightful place and set things right.
He might not even realize what he is doing because of the grief and pain.

Just don’t let yourself get caught up in a relationship where you are trying to do all of the accommodating.
You deserve a fresh and fulfilling love relationship where it is about YOU and your love.

I believe you know the answer to what you want but are afraid that you might seem selfish to pursue it.

If you aren’t 100 % in love and sold out, then relationships are really hard and will take a toll on you.
Give some though to the questions: Is my life (right now) better or worse with him? Is he meeting my needs? (You have said no to this one.)
Think about what you WANT now in your life at this time.
Being 25 is a wonderful thing. It is a time when we sometimes will choose something that will stay with us for many years or even forever, so choose wisely. Please don’t SETTLE.
I want you to have fun and feel energy and hopefulness at this time of your life.
Please let me know how things go.