ماذا يفعل الشاب مع صديقه وأخت صديقه بعد أن تعلق قلبه بها؟

Regarding your friend, you are obligated to respect his rights by helping him in good deeds, reminding him of Allah, teaching him, and protecting his honor. As for his sister, she is a non-mahram, and it is impermissible to be in seclusion with her or to converse with her. If you wish to marry her, you should approach her guardian.