why is this wrong? você ama a minha irmã?
It's accepted without any change in meaning.
put the same, am also wondering why the "a" is not accepted
Would it be possible to use the verb 'querer' instead of 'amar', like Spanish?
No, you couldn't. Despite the verbs "querer" and "amar" being synonyms in Spanish, in Portuguese "querer" means "to want".
It seems that no. I use and it was marked as wrong.
Why did reject "você amam minha irmã"?
Você ama (singular)
Vocês amam (plural)