There should be bisexual on tags
I don't think it's clear that either of them is bi?
Little sister is labeled as "experienced", it's even in the title, rofl, and is even dragging poor "oneechan" to a love hotel like its nbd, but also plainly states this would be her first girlfriend. So at least bi. Not bothered by the lack of tag tho
Just cuz she is "experienced" it doesn't necessarily mean that she's "experienced" with having sex with boys.
And also, just cuz she would be her first "girlfriend" it doesn't mean that she's isn't "experienced" with girls.
So we can't be completly sure that she's Bi or not.