Learn Spanish (Latin America): Las hijas del doctor son simpáticas.

Spanish doesn't use an apostrophe 's' to show ownership. Instead, you flip the phrase around to say "the daughters of the doctor" using the preposition de (of).

Why does the sentence use del instead of de el for "of the"?

In Spanish, when the word de (of) is right next to el (the masculine word for "the"), they must combine into a single word: del. So, de + el doctor always becomes del doctor.

The doctor is masculine, so why is the adjective at the end feminine (simpáticas)?

Adjectives must match the person or thing they are describing. Here, the word simpáticas (nice) describes the daughters (las hijas), not the doctor. Since las hijas is feminine and plural, the adjective must also be feminine and plural.

Why do we use son for "are" instead of están?

We use son (from the verb ser) because being a nice person (simpática) is considered a core personality trait. The other verb for "to be", estar, is used for temporary states, current feelings, or locations.