Why twins?

I thought it was sadder that the wife had two babies and, like she says to her husband, could easily have more because she'll never have to struggle to raise them. Made it all the more unfair to the maid. But I agree with you that in general twins can seem pretty played out.

With the wife that far into a pregnancy with twins makes it even more urgent that they need a new employee to care for her as well as everything else.

The two kids amplifies how much more the wife has than the housemaid.

SPOILER
Having two new babies and still wanting the housemaid to have the baby shows a side of the husband that is both tender and selfish (it's all about him).

The fact of twins is a foreshadowing of how nasty and duplicitous the family are. You advertise for a nanny to take care of one baby, but actually there will be two. That's a big difference and ought to have been disclosed in advance.