I've often heard that the Welsh "ap" means "son of" and/or "son" and is therefore roughly equivalent to "mac". That comparison instantly raises the question of whether there was any Welsh equivalent for the Scottish "nic". Since "mac" and "nic" seem tied to clan, it seems possible that there isn't, but I've often wondered and am not sure how to go about researching it.
According to my dictionary, "merch" is daughter in modern Welsh, while son is given as "mab."