Nicene and Post-Nicene Fathers, Ser. II, Vol. XIV:
The Canons of the Councils of Ancyra, Gangra, Neocæsarea, Antioch and Laodicea, which Canons were Accepted and Received by the Ecumenical Synods.: Canon II

Canon II.

If a woman shall have married two brothers, let her be cast out [i.e. of communion] until her death.  Nevertheless, at the hour of death she may, as an act of mercy, be received to penance, provided she declare that she will break the marriage, should she recover.  But if the woman in such a marriage, or the man, die, penance for the survivor shall be very difficult.

Notes.

Ancient Epitome of Canon II.

A woman married to two brothers shall be expelled all her life.  But if when near her death she promises that she will loose the marriage should she recover, she shall be admitted to penance.  But if one of those coupled together die, only with great difficulty shall penitence be allowed to the one still living.

It will be carefully observed that this canon has no provision for the case of a man marrying two sisters.  It is the prohibited degree p. 80 of brother’s wife, not that of wife’s sister which is in consideration.  Of course those who hold that the affinity is the same in each case will argue from this canon by parity of reasoning, and those who do not accept that position will refuse to do so.

In the Greek text of Balsamon (Vide Beveridge, Synod.) after the first clause is added, “if she will not be persuaded to loose the marriage.”

Van Espen.

The meaning of this canon seems to be that which Balsamon sets forth, to wit, that if a woman at the point of death or in extremis promises that if she gets better she will dissolve the marriage, or make a divorce, or abstain from the sacrilegious use of matrimony, then “she may be received to penance as an act of mercy” and surely she is immediately absolved from the excommunication inflicted upon her when she was cast out and extruded from the Church.  For it is certain that according to the discipline of the Fathers he was thought to be loosed from excommunication whoever was admitted to penance, and it is of this that the canon speaks; 123 but he did not obtain perfect reconciliation until his penance was done.

To this performance of penance this woman was to be admitted if she got well and dissolved the marriage according to her promise made when she was in peril of death, as the Greek commentators note; and this too is the sense given by Isidore.

Footnotes

Van Espen gives “fructum pœnitentiæ consequatur” as the translation of ἕξει τὴν μετάνοιαν.