What is the ruling on a man saying to his wife: "If you do not come to my family's house to spend Eid, then you will be divorced," if her coming does not materialize? Does the divorce take effect or not? And if it does, what are the rights of the wife who is six months pregnant?

The majority of scholars hold that a conditional divorce takes effect when the condition is met, regardless of whether divorce was intended or merely a threat. This differs from the view of Ibn Taymiyyah, who maintains that it does not take effect if the intention was to threaten, prevent, or encourage, and in such a case, an expiation for breaking an oath suffices. Your statement "you will be divorced" could be interpreted as either a promise or a conditional statement. If you intended it as a promise, then no divorce has occurred. However, if you intended it as a conditional statement, then divorce has occurred if your wife did not return before Eid. If the divorce occurred and it was a revocable divorce, then you may take your wife back before her 'iddah (waiting period) expires. As for her rights if you divorce her and do not take her back, she is entitled to her full Mahr (dowry), her maintenance, and her lodging for the duration of the 'iddah.