I hope you didn't get that information from Wikipedia - it's not authoritative, because anybody can post anything with or without documentation.
Looks like the Zouche tree is corrupted by bad information, too. There was a Geoffrey de Porhoet who married somebody named Hawise, but it was early in the 12th century, she was *not* the daughter of Alan "Fergant", and one of his *sons* became the first documented Zouche. Geoffrey's father was Eudes, first documented Vicomte de Porhoet.
Geoffrey's *brother* Alain (the family was far too fond of that name!) became Vicomte de Castelnoec (and possibly Rohan), probably by marrying the heiress (her first name, Villana, is on record, her ancestry is not). Alain's son Alain (see what I mean?) was the first explicitly recorded Vicomte de Rohan.
Cawley's Medieval Lands is in general a better guide to who was whom in the Middle Ages,because he accepts nothing without documentation and clearly labels speculation as speculation. http://fmg.ac/Projects/MedLands/BRITTANY.htm
