I've been looking at this. Turns out it was a family affair. When it came to finding a list of 25 sureties, the ringleader Robert Fitzwalter started with his own family and didn't have to look much further. His own 1st and 2nd cousins supplied more than half the sureties and the wives of several more.
His niece Maud Pecche was Lanvallei's wife.
His aunt Maud was Albini's mother.
His grandmother Maud de St Liz was grandmother of Quincy and Huntingfield by her 2nd marriage.
His great-grandmother Maud of Huntingdon became Queen of Scotland by her 2nd marriage and was an ancestor of Bohun and Ros's wife.
His grandfather was the brother of Gilbert fitz Richard de Clare. Gilbert's daughter Alice was ancestor of Vere, the Bigods, Lacy and Clavering. Her brother Richard was ancestor of the Clares and Say's wife. Their brother Gilbert fitz Gilbert was ancestor of Marshall jr and 2 more wives. Gilbert fitz Richard was also an ancestor of Percy, Montfichet and Forz's wife.
Many of the sureties had other cousinships by descent from the Bigod sisters, though Fitzwalter himself wasn't one of that set.