This dude is just so many green flags. Girl's luck is outstanding, finding someone this wholesome, respectful, and caked the hell up. Snap him up, sis (by which I mean, take your time, be his friend, roughhouse a little, and then ask him out in the middle of that - ideally while you're on top so he doesn't feel like he's pressing down on you and also so he can't back away too easily).
Not that sis doesn't have a number of green flags herself. Despite having ulterior motives to begin with, she actually enjoys spending time with him, tries to help him with his image, and properly communicates what she's feeling (aside from the bits about sex, of course).