Okay this is from what I understand about this manga, Mikage gradually developed feelings for Tomoe, and wanted to take their relationship as lovers instead of sisters. Tomoe on the other hand, is generally still hung up in the past, and wanted to somehow preserve the image of their family, even if its only with just Mikage, that is why Mikage accused Tomoe of imposing her ideology on her, and is protecting Mikage to indirectly protect herself.
As for the relationships:
Takayanagi and Mikage: Takayanagi is in love with Mikage, and Mikage will eventually know. Takayanagi's love may be unrequited but Mikage realized that Takayanagi is precious to her, that prompt her to sought advice from her father regarding her situation with Takayanagi.
Kujou and Tomoe: Kujou is in love with Tomoe, I think Tomoe unknowingly developed feelings for Kujou, this is just my opinion, since Kujou had been so nice to her and she somehow fits Tomoe's image of security. The problem is Kujou's way of thinking about love and her twisted way of expressing her feelings for Tomoe.
Tomoe and their father: Tomoe hates their father after their mother died, because he became irresponsible and was too wrapped up with his art, that he neglected taking care of his daughters. There was also an implied incident that he did to Mikage which was not clearly discussed in the manga, either it could be an abuse or traumatic event for Mikage. Tomoe absolutely hasn't forgiven her father about that.
Mikage and their father: Mikage has forgiven their father about what happened in the past, or just plainly don't care about it anymore. She always says that his art is beautiful that's why she still visits him. I think due to whatever happened 2 years ago when Mikage was 13 years old or even years before that, that created an agreement between Tomoe and Mikage, that Mikage will not go alone to visit their father, maybe the incident started when their father called Mikage as "Kaede" their mother's name, since she resembles her mother, It could be that their father forced Mikage to pose as nude before? But didn't exactly raped Mikage, since Mikage's first kiss was with Takayanagi. But I think that their father wanted to have an incestuous relationship with Mikage, based on the way he looks at Mikage with those eyes, but was controlling himself, he also said that Mikage lets him remembers the feelings that he used to have, and he also said that he looks like Tomoe before puberty, that they are alike, he seems to be implying that he can be good enough substitute for Tomoe, to which Mikage replied that they don't resemble each other, and he's not the one she needs in her life. He also speaks his mind loudly about being jealous towards Tomoe, maybe because Mikage chose Tomoe instead of him.
Tomoe and Mikage: Now the relationship between Tomoe and Mikage, if you will look back one of the reason why Tomoe can't accept Mikage's feeling was because Tomoe still can't accept change, and still clings to her image of family, she also confessed to Natsuo that she doesn't understand "love" since she haven't been in love before and that if ever she understands love, she's afraid that things can never go back the way it was before, so she refused to acknowledge her feelings for Mikage, even if it clearly shows that she's also having the same feelings for Mikage, being flustered and concious around Mikage.
I honestly think that there's a progression between Tomoe and Mikage's relationship, the vacation after Tomoe's graduation seems like a "honey moon" for them, in the final chapter you can see the dynamics between them, and there's no more tension like before, just serene conversation, so I think Tomoe already accepted Mikage's love, and were already a couple. I think the blooming algae symbolizes their relationship.
