Try not to get too caught up in these things. However have you expressed to your family that you would like to be there? They may think that since you have grown up and moved into your own life that you may be busy or something. Maybe talk to your Halfbrother and let him know what it would mean to you o be invited?

Step families are a tricky thing. My Dad and Mom divorced when I was young. I am 46 now. There were 4 of us. My dad went on to have a second family. He adopted step moms child and then had two more children. So now I have 2 half sisters younger tham my own children and a adopted step sister that does not even have the same family as me. ( she still sees her biological dad ?) I have met him once.
My Step moms parents were nver close to the original 4 kids and so we were just those "other kids " so were not realy invited to family stuff on the step side. The part that got me upset was when we stopped bing invited to holidays and my dad only brought the new family. Years later we do not have much of a relationship in fact the whole thing is just downright confusing. So we consider ourselves the original 4 family members plus our biological mother, and we have get togethers all of the time. My dads kids have their own family things and then we invite each other for the big things like weddings etc..
The problem is with the new " family" structure no one even knows what the etiquite is so I guess we just have to tell eachother what we need to feel part of whatever family we belong to....