I should paraphrase Isabella's remarks thus:--

"If it be otherwise, if we are not all frail as thou sayest,
then let my brother die, unless he be but in the same case as
others; if he alone possess and follow thee in that particular
frailty to which thou has half confessed."

A feodary, I should observe, was an officer of the Court of Wards, who
was joined with the escheator and did not act singly; I conceive
therefore that Shakspeare by this expression indicates an associate; one
in the same plight as others; negatively, one who does not stand alone.
In _Cymbeline_, Act iii. Sc. 2., we read:

"Senseless bauble,
Art thou a _feodary_ for this act, and lookst
So virgin-like without?"

where feodary clearly means confederate, associate. According to some,
the word signifies one who holds land by the same tenure as the rest of
mankind; whilst Mr. Knight, in a note on _Henry IV_. Part i. Act i.
endeavors to show that it includes both the companion and the feudal
vassal.

"To owe" is frequently used by Shakspeare in the sense of to possess, to
own, as in Act i. Sc. 5. where Lucio says:

"But when they weep and kneel,
All their petitions are as freely theirs
As they themselves would _owe_ them."