I'm sure the ideas others here have suggested will work well for you.
I just wanted to commend you for trying to address the sibling situation, and I wanted to extend my condolences for the passing of your brother. I lost my sister (8 yrs older than I) 3 yrs ago, and the grief still hits me hard. Which leads me to the point I wanted to share. I was the youngest of 4 kids: my sister +8yrs, a brother +8 yrs from her, and our eldest brother who died when I was a toddler. I never really knew my brothers, but I worshiped my sister as your little guy does his brother. She wasn't interested in playing with me, especially as she got older, and I do remember her being mean to me when we were young.
But after I became an adult, we became close, and closer still after we both had children. All of which is a long-winded way of saying, don't despair - if your efforts at encouraging your older son don't immediately pay off, they have a life-time ahead of them to create a relationship. I know it's sometimes hard for me to see the "big picture", especially having lost a couple of siblings and knowing that opportunities can be lost at any time.
I also have step-children (half-sibs) who are much older than my own two. The relationship between your sons, just like any relationship, will continually evolve. Having said that, it might be helpful to note that sometimes, no matter how much positive influence we try to have, our children's personalities just clash. You were lucky to have a close relationship with your brother (and I know that must make you yearn for your sons to share in that), but (I'm reminding myself here) sometimes siblings just don't get along super-well. I guess the best we can do is to foster respectful interactions regardless of personal feelings.
Another of your posts spoke of the difficulties with your 10yo - I empathize. It sounds like you are trying so hard to be the best mom you can be - my heart goes out to you. I like to think that our *intent* goes a long way. Parenting with our hearts, imho, is more important than "perfect" results. Trying to control outcomes can drive us nuts! In an effort to ease the universal feelings of "mother-guilt", I ask you to show as much kindness to and appreciation of yourself as you do your sons.
Best wishes, and congrats on being such an involved mom.