Someone exhibits "crab mentality" if they try to drag people down who are doing better than themselves. A form of "If I can't have it, neither should you" thinking.
Wikipedia says that "the metaphor is derived from a pattern of behavior noted in crabs when they are trapped in a bucket. While any one crab could easily escape, its efforts will be undermined by others, ensuring the group's collective demise."
For example, if one person from the family succeeds, the other siblings may become envious and try to undermine their success. A similar effect may occur in friend groups, for example if one friend gets a new job and starts earning twice as much as the other friends.
More lethal examples come to mind as well. Consider this all-too-common story: a man and a woman break up. The woman commences a new relationship. The man, now bitter, kills his ex-girlfriend because "if I can't have her, no one can".