I do not think Prime Ministerial spouses should play any part at all in political life. I do not even want to see happy family snaps on election leaflets.
You vote, or should, for an MP on the basis of their party affiliation and personal views on specific issues, not because their spouse is good looking/will run the constituency office/dress smartly for royal occasions. For once I admire Jeremy Corbyn. I know he is married, has had more than one wife, has sons - and that is it. I assume his wife leaves him to get on with his job and she gets on with hers. ditto their sons, none of them seem to be trying improve their standing by riding on the 'glory' of his position. Samantha Cameron played no part in her husband's political life other than as an accompanying spouse at big events, and as far as I am concerned that is as it should be. She has a life and career of her own.
The USA has a completely different electoral system. the President runs on a party ticket but runs as an individual - hence all the primary elections before the choice is made. The wives of presidents have often been strong women and unofficial vice-presidents. Think Martha Lincoln, Eleanor Roosevelt, Jacky Kennedy, Rosalind(?) Carter, Hilary Clinton. Michelle Obama has just been following in a grand tradition of presidential wives.