I was the only child of my mother and father; my 4 older sisters were from my mother's first marriage. My mother had not intended to have another child with my father...but I happened. On top of that, my mother and father divorced.
The biggest issue growing up was the two separate families… and it's still an issue, though more to me than my sisters. Their father's big family always had reunions and parties that I didn't go on because my father didn't want me off with their family and father or my father had visitation that weekend. This always made me feel left out and like a half sister. It was more my father's doing at the time but this trend has continued until today. Every time my sisters discuss their latest family reunion or how interesting it is to find out something about their ancestry, I feel totally shut out and ignored. They don't think about it probably, but as an only child (in a way), I don't have that camaraderie with another sibling that they do.
My mother also often had to deal with me differently than my sisters because my father was very verbally abusive. My sisters would point out she wasn't as hard on me as she was on them. With blended families, you can't treat everyone equally because each kid from another family has another group of family members and father/mother so each kid needs to be treated differently: it doesn't always come up fair or acceptable for everyone.