Hey Everyone,
I was watching the Tudors (don’t judge me!

) and the King had one of his marriages annulled on the basis of him having carnal knowledge (aka, had slept with) his wife’s sister before they got married. Does this still render a marriage null and void?
This is an impediment of affinity or public propriety. It used to be called just affinity, but now there is a distinction between affinity (relation by marriage) and public propriety (a non-marital, carnal relationship)
The impediment can be dispensed.
Henry’s marriage Catherine of Aragon (who was his brother Arthur’s wife, although never consummated) had an impediment of affinity-- the Pope gave a dispensation prior to his marriage to Catherine of Aragon (which is why his petition for nullity on the grounds of affinity was rejected).
Ironically, he applied for this same dispensation to marry Ann during the proceedings to have his marriage to Catherine declared null because of his relationship with Mary Boleyn (with whom he had an illegitimate son). He actually received that dispensation, although it was predicated on his marriage being found null-- which of course it wasn’t.
Secondly, if it does nullify the marriage, is a married couple that finds out that this nullifies their marriage obligated to seek a decree of annulment or can they be allowed to remain married?
A married couple would know they have such a relationship prior to marriage, and could seek to have it dispensed. If they somehow married without a dispensation, then they can still get one at the time the affinity/public propriety impediment becomes known.
Thirdly, if a couple is planning on getting married and finds out that this would nullify their marriage, are they obligated not to get married?
They can petition for a dispensation from the Bishop.
Here’s an article on Affinity, although from the 1917 code of canon law (so it’s slightly different now with regard to degrees):
httpyavww.knight.org/cathen/01178a.htm
current code Canon Law:
Can. 1091 §1. In the direct line of consanguinity marriage is invalid between all ancestors and descendants, both legitimate and natural.
§2. In the collateral line marriage is invalid up to and including the fourth degree.
§3. The impediment of consanguinity is not multiplied.
§4. A marriage is never permitted if doubt exists whether the partners are related by consanguinity in any degree of the direct line or in the second degree of the collateral line.
Can. 1092 Affinity in the direct line in any degree invalidates a marriage.
Can. 1093 The impediment of public propriety arises from an invalid marriage after the establishment of common life or from notorious or public concubinage. It nullifies marriage in the first degree of the direct line between the man and the blood relatives of the woman, and vice versa.
Can. 1094 Those who are related in the direct line or in the second degree of the collateral line by a legal relationship arising from adoption cannot contract marriage together validly.