I am so sorry for your loss. I lost a sibling too, and had a similar reaction from many people.
After many years of reflection I think it is because losing a sibling at a young age, - even though you are an adult it is still a young age because you normally expect to have your siblings until you are both old - is relatively unusual , people do not relate to it in the same way as they do to say losing a parent or a grandparent, which many more people experience and understand, and which lets face it, is the natural order of things that we expect. Frankly most people do not have the social niceties and responses to hand to deal with the death of someone's sibling .
Also, in my situation the sympathy was , quite rightly, directed towards my sibling's spouse and young children, who were the focus of everyone's support, and I have absolutely no issue with that, it was and is right and proper.
But, I think people assume that as an adult you can cope with loss and grief, especially if people more vulnerable are also grieving, but it isn't actually so, the grief is the same, but the grieving is different , as is the loss, since you are losing the other half of a lifetime of shared childhood and adolescent memories , jokes, experiences , secrets, and family traditions which no one else possesses, as well as the possibility of future experiences and memories which have now been lost.
Again, condolences on your loss, it will slowly get easier. One thing I would say is if you have photos, videos etc make sure you annotate them as fully as you can, so the memory is strengthened and the photos and videos have more meaning for others as well as for you.