"Don't say it so bluntly."
Some things you've got to hammer in.
Sis got a good plan. Ask a favour that let's you hang out a bit, and return it with a date.
"Could it be that she didn't make the connection?"
Yes, but if you let it sink in, no.
Date advice she gives is also good, and her only mistake is that Yumizuka isn't going to be put off if he goes a bit hard. She'll probably get very embarrassed, though, which may or may not lead to something hilarious.
Both of them wear nice clothes. Personally I prefer a more flowy skirt, but that's fine.