I found an article today about a niece who was excluded from a wedding because she wasn't "real" family. She's a child from a previous marriage and was excluded from being a FG, let alone invited, to her step-uncle's wedding. But her younger half-sister was.
I don't come from a blended family, but my FH has an older half-brother. He was the child of a relationship between their mom and former boyfriend. My FH's fraternal grandparents always treated him no different than my fiancé. They were always treated as brothers because they ARE brothers.
You can read the story here: