Gotta add my membership in the "Little bro as an annoying rival is a NO!" club. I'm not against having other characters introduced, but how about using them in a non-cliched way? For instance:
1) Little bro recognizes what's up & decides to become a Lovemaster for his slow-moving senpai, giving a few good & a lot of not-so-good pieces of advice;
2) He sees his sister acting like a B-29 wingwoman & out of irritation at her heavy-handed interference tries to get her to lay off;
3) He decides to be Alice's wingman & tries to advise her on how to win the slow-burning heart of a young Japanese guy despite having even less effective communication synergy with Alice;
4) He acts as the tsukomi character who stands to the side & comments on the love comedy.
Any of these would be something different from the headache-inducing love rival trope.