Questions & Answers about Mis abuelos son muy religiosos.
Why do we use son instead of están for being religious?
Since abuelo means grandfather, does mis abuelos only refer to two grandfathers?
Not usually! In Spanish, the masculine plural is used to describe a mixed-gender group. So mis abuelos almost always means "my grandparents" (your grandmother and grandfather). You would only use mis abuelas if you were specifically talking about two grandmothers.
Why is the adjective religiosos plural here?
In Spanish, adjectives must match the noun they describe in both gender and number, even when they are separated by a verb. Since mis abuelos is masculine and plural, the adjective describing them must also be the masculine plural form religiosos.