"castcest" is sexual slang. In short, it refers to romance or sexual activity between members of a cast. Among teens it is usually used jokingly, as crude banter, or in TikTok-style algospeak rather than as a serious statement of behaviour. Persistent or aggressive use directed at a specific person, especially online, is the bit worth paying attention to: that can shade into harassment. If you hear it once in a chat with friends, it is probably nothing. Repeated targeted use deserves a conversation about respect.