I think you answered yourself, childhood stuff can linger a long time and likes to hide under the surface…
I have an older sister, not much more in common but our parents, but we get along the one or two times a year we meet, dad is taking up most of the space most of the time anyway so whatever. 
Also people have all kinds of expectations of us, that have nothing to do with us…
Be nice and don’t talk too much has been a strategy that worked well for me.