I think you are right.

Henry VIII ad his first wife Katherine, after years of marriage had only a son who lived 2 months and a daughter, Mary.

Around the time he met Anne Boleyn, Henry claimed his conscience was troubling him as Katherine had been briefly married to his (then dead) elder brother.
There was a Papal dispensation to enable Henry and Katherine to marry but Henry claimed doubts as God had not seen fit to give them living sons.

Manfred tells Father Jerome 'It is some time that I have had scruples on the legality of our union: Hippolita is related to me in the fourth degree- It is true we had a dispesation; but I have been informed that she had also been contracted to another. This sits heavy at my heart: to this state of unlawful wedlock.'
(ch. 2)

Similar to Henry VIII, Manfred appears to have a convienent conscience.
Manfred determines to dispose of Hippolita and acquire Isabella in marriage.