Is it permissible for a married man to supplicate to Allah to unite him on the Day of Resurrection with his wife and her unmarried maternal aunt (if she remains so)? And what is the ruling on this supplication?

It is forbidden to combine a woman and her maternal aunt under one man in this world. As for the hereafter, we have no knowledge of that. A Muslim should occupy himself with what benefits him and ask Allah for entry into Paradise, where he will find what no eye has seen and no ear has heard.