Actually, it doesn't surprise me that you have not had a relationship yet, that was never something you got a chance to have if you look back on how your younger brother needed more than you did and your mother had to tend to "him" more.

Then your mother had "two" boys and probably did not spend "quality" time with either one of you so that is missing for "both" of you which is why neither one of you have had relationships.

I think it's important to take a step back and consider the dynamics of what your early imprinting was, not just for you but also for your brother. Two years can really mean a lot in very early childhood and how a mother handled that challenge. It sounds like your "younger" brother always got to be "the baby" and that began for you when you were a "baby" yourself always waiting or being expected to behave while your mother saw to his needs simply because he was ALWAYS younger. SO, this sick of being valued the least most likely goes all the way back to when you were just a toddler/baby yourself and developed that feeling.