Sir Alan fitz Brian and Agnes fitz Henry did have a second son, Theobald (fitz Alan). He's briefly mentioned in Moore's Knights of Edward I, II:29 as being heir to his brother Brian. His heirs, in turn, were his nieces Agnes and Catherine, both daughters of Brian and his wife Maud (d. aft. Apr 10 1340). But the date of death for Theobald may be off by a couple of years as he still held Stowe manor on 26 Feb 1308 (inq.). Also see CP V:393-398.