Answer:
Hi, I just wanted to say that I got this question a few days ago on my test!
I haven't gotten my grade yet and I dont know if I got it right but I hope you figure it out! Best of luck to you!
Mon père est fort (masculine singular as the noun "père")
ma sœur est forte (sœur feminine ----> forte)
Ils sont forts (ils = 3rd person masculine plural ----> forts)
elles sont fortes (elles = 3rd person feminine plural ------> fortes)
the adjectives , even after a verb , (adjectifs attributs du sujet ,in French) do have to agree in gender and plural or singular.
Hope I helped !!