Aki knowing about Shoichi and MC's relationship determined on if on Day 21 you "Tell the truth" (i'll refer to is as route A) to Saya or "Deny it" (route B). I'll try and compile all consequences of this choice:

On Day 22 Aki during conversation with MC he'll either:
A: He'll ask for advice on how to tell a girl if you like her.
B: He'll asks (or rather confirms his suspicions) about MC and Shoichi dating.
Issue: Later that day Shoichi, after talking about how happy he is that MC told Saya the truth (A) or being sad about MC lying to Saya (B), MC tells that Aki knows no matter if you had the conversation about that with brother or not, which i guess is a bug. UPD: A and B scenes should be switched, considering information prodided in my second post.

On Day 25:
A: During conversation about the fight MC with Shoichi's father, MC debates if he should tell Aki or not, but decides against it.
B: During the conversation mentioned above MC debades if he should talk about his problems with Aki, even if Aki knows about Shoichi and MC.
UPD: A and B scenes should be switched, considering information prodided in my second post.

On Day 29(A, where i pushed Sho to reunite with his mother during Day 26) / 28 (B, where i didn't push Sho to see his mom during Day 26) Aki and MC have a conversation in the morning, when they are going to school together:
A: When talking about Aki spying for Hitoka, he says that MC should tell her about dating Shoichi, as well as telling mom.
B: MC mentions the scholarship offer to Aki. Brother thinks that MC should tell mom about it.
Issue: I feel like these two conversations should be switched, since during route A, MC and Aki didn't have the conversation about MC dating Sho yet. Or, perhaps, scene during day 22 should switch, since (if i remember correctly) MC learns about Aki spying for Hitoka only in route A (when Shoichi sees his mom) and C (when MC comes to Hitoka's classroom, after the fight with Shoichi).

Day 31 (A) / 30 (B):
A: MC debates if he should tell Aki about dating Shoichi (again), but interrupted when Sho knocks on the door. Later, when Shoichi helps clean the kitchen, Aki finally confronts the couple and Sho admits that they are dating. Woriking as intended, i think.
B: MC doesn't want to tell Aki about the date and MC and Sho being a couple (even if Aki already knows?). Then MC debates if he should talk about the relationship problems with Aki, since the little brother already knows. Then he debates to if he should tell Aki about dating Shoichi (again? and Aki already knows?!), but is interrupted by Sho knocking on the door. When Shoichi helps MC with cleaning the kitchen, Aki confronts the couple just like in route A.
Issue: The entire scene in Route B.

On Day 34(A)/33(B):
A: After Aki argues with MC and Shoichi should tell their families about dating, Shoichi says that he forgot that MC mentioned to him (during day 22, which is a bug?) that Aki knows about them dating.
B: Same scene as route A, but i guess it works as intended in this scenario (but doesn't make sense, if Aki confronted the two in the previous day?).

I might've missed something else, since i got tired already, and i didn't test route C, since its way too different.