I suspect any restrictions on this pairing could come under TEM philosophy. There will never be a definitive right/wrong answer. It'll be 'opinions are like rear orifices; everyone has one'. It can be argued, and not completely discounted, that there is a risk of familiarity, family feuding, cockpit gradient, who's in charge, slack oversight to avoid confrontation that has to be endured for days etc. etc. It could also be that all is in the rose garden. If a DFO does not want to take the risk then no problem.
If it considered a near certain disaster to try & teach your wife to drive why would it be considered strange to avoid being locked up in a flight deck for hours when one of you is the ranking boss.
I used to fly, occasionally & in 2 different companies, with my wife as purser. Service was impeccable and it gave us a crew party room on night stops, or a snore escape boudoir. Once we arrived home she said, "uniform off and now we are in my territory."
I'm sure that somewhere, sometime, probably the USA, there was a crew with Dad & Son up front and mama ruling the roost in the cabin. It's the sort of thing that might be organised for Dad's last flight. Those in the know.........