Muchas gracias a todos. Me han ayudado mucho!

"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

mi hermana mayor, mi hermana menor. Comparative adjetives don't have gender. mi hermana mejora = my sister gets better.

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 :)