It seems like it's not so much that she doesn't care as it is that L'il sis has an indestructible innocence that's completely missed that her big sis want to have her for breakfast, lunch, dinner, and dessert.
I think I need to re-read this all in one go- I was getting confused keeping track over who was who and who likes who.
Shima is big sis. She loves her little sister. Mai is little sister. She's utterly innocent and hasn't figured out that her sister wants to fuck her senseless. Yuu is Shima's friend. She has a crush on Shima that Shima has completely missed because she's obsessed with her little sis. Kozue is Yuu's little sister, and Mai's underclassman. She is majorly in love with Mai, and has confessed but been rejected, which didn't faze her in the slightest.
Mother is Shima and Mai's mom, who is a yuri mangaka who gets off on the thought of her daughters being in that kind of relationship.