"У сестры уже ребёнок."

Translation: Sister already has a child.

[BenGreenacre]

Is there a specific reason why 'есть' is omitted?
I.e у сестры уже есть ребёнок.
Is that a wrong answer?

[ZoeGrimes]

You can either include есть or omit it, so including it isn't wrong.