Is the mother's maternal aunt a mahram to a man? And is the mother's maternal uncle a mahram to a man's sisters? And is it permissible to wipe over a sock after removing the sock that was on top of it, then putting it back on after the wudu' is broken, while the second sock remains?

The maternal aunt is among the unmarriageable women mentioned in the Quran. There is no difference between the maternal aunt of the mother and the maternal aunt of the grandmother; every maternal aunt is unmarriageable, no matter how distant the relation. As for wiping over the socks, it is not permissible to wipe over them if one of them was removed and then put back on, because the condition for wiping—which is wearing them after a complete state of ritual purity—has not been met.