His choice made Simon an Englishman, but his foreign blood still moved the jealousy of the barons, and this jealousy was quickened by a secret match in 1238 with Eleanor, the king's sister and widow of the second William Marshal.

The match formed probably part of a policy which Henry pursued throughout his reign of bringing the great earldoms into closer connexion with the Crown.
That of Chester had fallen to the king through the extinction of the family of its earls; Cornwall was held by his brother, Richard; Salisbury by his cousin.

Simon's marriage linked the Earldom of Leicester to the royal house.

But it at once brought Simon into conflict with the nobles and the Church.

The baronage, justly indignant that such a step should have been taken without their consent, for the queen still remained childless and Eleanor's children by one whom they looked on as a stranger promised to be heirs of the Crown, rose in a revolt which failed only through the desertion of their head, Earl Richard of Cornwall, who was satisfied with Earl Simon's withdrawal from the Royal Council.