Spoilers for Rumpelstiltskin would be nice. It's been pretty much
spaded on the forum.
Looking through the portal will display 3 messages. Two for one parent and the third is for the other.
Document with all messages here. The tricky part is that a message like, "cutting a huge piece of cake to eat alone" could be either Greed or Gluttony. It does not indicate both so there is a 50/50 chance of each. Other messages like "studying a treasure map" indicate only one sin so that is definite.
It would be nice if when you talk to Father/Mother/Both there was a spoiler indicating the chance of each sin being the correct answer. It's obvious for Father/Mother, but trickier for Both. The trick is that for talking to both parents, the sins are calculated separately from the individual parents. To explain how it works for Both parents it is helpful to have an example:
[table="class: tborder"]
[tr][th]Parent[/th][th]Messages[/th][th]Indicated Sins[/th][/tr]
[tr][td]Father[/td][td]writing a contract to make a high-interest loan[/td][td]Greed & Greed[/td][/tr]
[tr][td]Father[/td][td]checking his reflection in a spoon[/td][td]Gluttony & Vanity[/td][/tr]
[tr][td]Mother[/td][td]shining a golden chalice to a reflective finish[/td][td]Greed & Vanity[/td][/tr]
[/table]
Father's Sins: Greed (100%), Gluttony (50%), Vanity(50%) -- Father is definitely Greedy, but he is also either Gluttonous or Vain.
Mother's Sins: Greed (50%), Vanity (50%) -- Only one sin is valid
Both: Greed (50%), Vanity (33%), Gluttony (17%) -- Only one sin is valid
For both parents, the chance of a sin appearing is the number of times each sin is named divided by total number of sins.
It is possible for the Father to be Greedy and Gluttonous, the Mother to be Greedy, but when talking to Both together the correct answer can be Vanity.