The correct version is اختي دائماً مشغولة with a ة. Duo accepted it, but told me I have a typo and corrected it to مشغول. This is incorrect. The subject is "my sister", therefore "busy" must be in feminine form.
To be clear, they allow دائماً to come either before or after مشغولة, however they use masculine form if you put دائما first. It should still be in feminine form regardless of its position in the sentence.