There are two options in these copula (binding verb) clauses with a noun predicate (something is something). You can use the nominative or the instrumental. We normally accept both but an advanced student can learn that instrumental is mainly used for temporary states or for something one can become. You can become a father so the instrumental case works well here. If the predicate only includes an adjective, one should just stay with the nominative case (To auto je modré. Ona je pěkná.).