Homie just man up and tell your boy you're feeling his sister. Let him know that you're gonna treat her right and not like a fuck buddy since there's obviously feelings involved. If you're a man of character your homie knows this and will give you that blessing. After that, her know how you feel.
Let your boy know what's up first before you tell her sister what's up.
ya ima do it this over weekend wen i see him . i think he will approve me and her being together