Llanthony Priory had been established near Crucorney, in the Vale of Ewyas, in 1118; Wales' earliest Augustine monastery. Miles' father, Walter de Gloucester, had retired there by 1126. The unrest that had been simmering in Wales during the last years of Henry's reign, boiled over in 1135 on his death. The area around the priory returned to Welsh rule, coming under such “hostile molestation” from the Welsh that the non-Welsh canons decided to leave. Miles established a new Priory for them in Gloucester, England, which they called Llanthony Secunda, in 1136. Sometime after 1137, Sibyl, together with her husband, made a further endowment to Llanthony Secunda.
