[sigmoid10]

Purely guessing here, but the closest connection a one and a half year old usually has is its mother. Fathers only become more "relevant" to them at 2+ years and people outside the family even later. So it would be normal for the kid to pick up the mother's tongue. But it would be interesting to see how this preference evolves in the coming years.

[irrational]

I'm not sure that is it. The mother is an engineer who works a typical 9-5 job in an office and often travels for work. The father works remote and is home all day.