Family Lore Clinic: In the UK is the mistress ever required to pay maintenance?
Well, I can only speak for England and Wales, but the answer is simple: no. A 'mistress' (which I take to mean a woman who is in a sexual relationship with a married man) can never be liable to pay maintenance to that man's wife, or for his children.
To give a little more detail (and I will only deal with spousal maintenance, as child maintenance is normally paid via the Child Support Agency), a court cannot order a 'mistress' to pay maintenance to the wife. She is not a party to the proceedings, and the wife cannot claim maintenance from her.
However, if the wife claims maintenance from the husband, the court will take into account the husband's outgoings. If he is living with the 'mistress' and the mistress is working, then that may reduce the husband's outgoings, and therefore increase the amount he has available to pay maintenance for the wife. In those circumstances, the husband could be ordered to pay more maintenance than if he were not cohabiting.
If you require any further details regarding the issue of maintence, then you should consult a specialist family lawyer.