Yuu is not entitled to a relationship Hikari. Hikari, although she loves Yuu, seems to understand that currently a relationship with Yuu would be toxic and filled with misgivings. Perhaps if Yuu matures then they can rebuild their relationship.
But the sad weirdo is her friend which he was kissing the previous day. If he dated a random girl a year ago then sure, but the person and recency is the issue.
Key's Kanon, one of the major romance VNs, has the ml's first cousin as a route. Since the original version is an eroge, we see him "consummate" his relationship with said first cousin. Not as,"bad" as sister, but still close family.
My college chemistry department was this lax back in 2010s. We lit thermite in the parking lot as part of the teaching lab. High schoolers can occasionally join college research projects.
Please don't use fluorine as an oxidizer for organic compounds, there are too many unknown PFAS in our water ways already. 😅 Or perhaps do as it might lead to my PI getting another grant to study their removal. 😆