A hypothetical question. The situation is not mine (discussed with a friend over the weekend), but it got me thinking.
So the situation.
A single mother at the moment is a sole provider for her young children, and has been from their birth (dad is not in the picture financially). She is in a relationship with a man, which gets serious and the next logical step is marriage.
Her new partner wants to have children in this new marriage (he doesn't have any), and she is not averse to the idea herself. Which will, most likely, lead to a couple of years of decreased earnings for her (her job is not very compatible with caring after babies / toddlers). No one is on the breadline here, but not wealthy either - at the moment she probably earns a little bit more, but it is quite situational, their earning powers are roughly equal.
What is the "fair" way to deal with costs regarding her older children in this case? I am genuinely interested in all opinions, of course in every specific case the arrangements would be unique to the family. I am just interested in what people consider to be fair in the abstract case. Should her new DP pick up some of the costs in respect of the stepchildren or not?