Of course, people w/siblings have an underlying jealousy, even animus towards people who are only children. Also, the prevailing prejudice against only children come from people from large/very large families of 6 children or more. People w/siblings cannot fathom the world of only children. In their purview, they contend that every child should have siblings because they do. They furthermore contend that only children are "missing out" because they do not have siblings.
People w/siblings see only children as incomplete people.They do not understand how children do not have siblings. Then they misperceive only children on the basis of the latter's onliness. They associate only children w/ being lonely, spoiled, and even selfish because the latter have no siblings. However, there is a subconscious envy. People w/siblings observe how happy, self-contained, and assured only children are in comparison to them.
People w/siblings are highly territorial and competitive. To them, such behavior is part of life. When they observe only children, the latter do not exist such a territoriality nor competiveness. They also observe that only children have an ease and comfort that they do not and will never have because there will always be a sibling or siblings who somehow threaten or disturb their safety, security and/or comfort level. What people do not understand, they misjudge from a sense of envy.
Yes, people w/siblings are envious of only children because only children have opportunities and advantages that they do not and will never have. Only children do not have to vie for parental attention. They have their parents' undivided attention and can go to them when they want to. Children w/siblings have to fight for parental attention. Also they have LESS parental attention. In some multichild families, some children will receive most of the attention while others will be cast aside, left to raise and parent themselves.
Only children have more monies allocated for them for a better quality of life in terms of socioeconomic, educational, and cultural advantages which oftentimes last through life. People w/siblings oftentimes have to struggle throughout childhood and beyond for educational, socioeconomic, and cultural opportunities. They know this and oftentimes take out their displaced anger on only children instead of their parents.