I have both "fictive kin" (what you call a "pseudo sibling") as well as extended siblings (siblings who never grew up with me, who are my half-siblings and foster siblings to my half-siblings who were in foster care) as well as an estranged sister and other estranged half-siblings who sided with my sister.

My fictive kin (pseudo siblings) include some of my veteran friends (veteran family). I'm not close to them as a sibling, but neither am I with any of my real family members. It's hard for me to get close, even though that's something I crave.

I, too, have fantasized about being an only child at times - but it was more in line with being rescued and taken away to foster care with loving parents who adopted me and visiting birth parents who apologized for their actions eventually. I fantasize about justice over all else. I longed for justice because I felt I never had any my entire life.