Is a husband permitted to prevent his wife from socializing with her male relatives (male cousins from paternal or maternal sides) at occasions such as weddings or births, even if she adheres to the Islamic guidelines, such as avoiding free mixing and dealing with them through their wives, simply because he is not psychologically comfortable with such socializing?

It is permissible to be courteous to non-mahram relatives, such as the children of maternal and paternal uncles, and the like, as long as the relationship adheres to the Sharia's regulations, avoiding mixing or seclusion. This is more appropriate, as they are among the relatives whose kinship is desirable to maintain; some scholars even hold that it is obligatory. You should be a support for your wife in this act of devotion, as its blessings are immense in this world and the hereafter. Repeated visits are not necessary to maintain kinship; rather, what is customarily considered maintaining kinship, such as visiting on occasions and inquiring by phone, suffices.