There is an inherent problem with the Corbet-Mallory line. Margaret (previously presumed Mallory) who married Robert Corbet, is now believed to have married William Mallory (Margaret's second husband) AFTER Robert Corbet's (her first husband) death. Primarily, the transference of property as a widow is what brought about this new insight into the Corbet-Mallory connection. Margaret Mallory has been shown to have been a "Mallory" only through her second marriage to Sir William Mallory. This makes Margaret the wife, not the daughter of Sir William Mallory. There has been some thought that she was Sir William's sister, but with the transference of property, it seems highly unlikely and can only point to Margaret's having married Sir William Mallory as her second husband. There are some excellent postings in the archives of the Genmedieval group (previously at Deja.com, but currently at Google,com. For the older postings, prior to 2000/2001 one must visit the Rootsweb archives, site posted below). This is an excellent source for medieval, and some later lines, genealogical study. There was a huge amount of discussion regarding the Corbet-Mallory issue from about 1996 to 1999 or 2000. Start with the year 1996 and move forward, using the search engine with key words like: Corbet-Mallory, Margaret Mallory, Sir William Mallory, etc. Follow the threads in sequence and you'll get the whole story. Plan on spending some time as there's a huge amount of discussion regarding it.
History of Parliament Online http://www.historyofparliamentonline.org/volume/1386-1421/member/co... does not commit itself as to the parentage of Margaret, but does say that she married Sir William Mallory (*not* an MP until, perhaps, after 1422) after Sir Robert's death. (Unfortunately the years 1422-1504 are not available.)
More about Margaret:
And some more:
and some interesting speculations from Hikaru Kitabayashi (to be considered, but cross-checked, as he tends to rely too much on guesswork): http://genforum.genealogy.com/mallory/messages/2746.html