From History of Parliament Online, entry for Sir Roger Corbet (d. 1395):

3rd s. of Sir Robert Corbet† (d.1375), of Moreton Corbet by Elizabeth, da. of Fulk, 1st Lord Strange of Blackmere. m. by 1382, Margaret (d. 14 Nov. 1395), da. of Sir Giles Erdington of Erdington, Warws.,1 2s. Robert* and Roger*, 2da. [3 extra daughters attached here] Kntd. by Mar. 1388.

http://www.historyofparliamentonline.org/volume/1386-1421/member/co...

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 (son of Sir Roger) 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 (son of Sir 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.

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Augusta Corbet's The Family of Corbet (Volume 2) (1914) is available online at http://www.archive.org/details/familyofcorbetit02corb.

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Erdington, Margaret

b. 1348 Moreton Corbet, Shropshire, England

d. 22 Nov 1395

Gender: Female

Parents:

Father: Erdington, Giles De

Mother: , Giles' Love

Family:

Marriage: ABT 1363 or nearby Shawbury Park, Shropshire, England

Spouse: Corbet, Roger

b. ABT 1330 Moreton Corbet, Shropshire, England

d. Jan 1396

Gender: Male

Parents:

Father: Corbet, Robert

Mother: Strange, Elizabeth Le

Children:

Corbet, Joane (Johanna)

b. ABT 1368 or nearby Moreton Corbet, Shropshire, England

Gender: Female

Corbet, Eleanor

b. ABT 1370 or nearby Moreton Corbet, Shropshire, England

Gender: Female

Corbet, Robert