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Adjetivos con nombres femeninos

You say: Mi mejor amiga
and : Mi hermana menor .

If you have any question, just let me know it. I'm willing to help you :)

mi hermana mayor, mi hermana menor.
Comparative adjetives don't have gender.

mi hermana mejora = my sister gets better.

"Mi amiga mejor" and "mi hermana menor" but it sounds better to say "mi mejor amiga", some adjectives like "mejor" or "menor" doesn't have a genre so i suggest you to search on internet a list of adjectives that don't have genre to learn them better