Is a young man obligated to accept his wife's maternal uncle's invitation to dinner, knowing that his mother refuses this and wants him not to visit his wife's relatives?

What appears to be the case is that you are not obligated to obey your mother in abandoning visits to your wife's relatives, because she has no valid purpose in this prohibition, especially if one of them invites you to visit. You should try to convince her, and if you cannot, then accept the invitation without her knowledge, as much as you are able. If she becomes angry with you, then strive to appease her and supplicate for her.