The Visitations are the direct *cause* of the mess, because they are *completely UNreliable* pastward of parish records. It's my personal opinion that the herald(s) assigned to this county spent all their time drinking in the local taverns and collecting local gossip, and NOT doing their job.
Other records (Chancery, land records, etc.) give Sir Roger Corbet *only two* sons, Robert (c. 1383-1420) and Roger (c. 1387-1430), both of whom wound up under the guardianship of John Burley, MP. No information exists as to any daughters (and the Corbet family was already multi-branched, which makes keeping track of them even harder). Burley arranged the marriage of Robert to a Margaret who must have been a close relative and was probably his own daughter (this was exactly the sort of thing guardians were prone to doing when they had underage marriageable daughters/nieces and underage wards).
Robert and Margaret had two sons, Thomas (died without heirs) and another Roger, and perhaps some daughters (not found in official records), before he died unexpectedly in 1420 during his term as Sheriff of Shropshire. Margaret promptly took as her second husband (Sir) William Mallory of Papworth St. Agnes and gave him at least one son (Thomas Mallory of Papworth St. Agnes - #2 candidate for authorship of the Morte d'Arthur).
As for the younger brother, Roger, he succeeded his brother in Parliament and is known to have married and produced at least one daughter (another Margaret) but no sons, before dying in 1430.
The brothers Corbet barely squeak in ahead of the 15th Century Pothole in the History of Parliament Online: http://www.historyofparliamentonline.org/volume/1386-1421/member/co... http://www.historyofparliamentonline.org/volume/1386-1421/member/co...
Roger Corbet the *brother* of Robert is often confused with Roger Corbet the *son* of Robert - but they were two different people.