>>531686345
Ze Amin and Deer Note are literal sisters. It refers to the two of them as Rising Scale's sisters, but doesn't say anything about whether Rising Scale is their sister or brother. But also she's obviously just a girl, and they even censored her chest for the TCG art so there's zero reason to think otherwise.
It's also possible that they're just calling Ze Amin+Deer Note her sisters metaphorically since it's extremely common to use "onee-san" to be exact they use 姉たち in the reference art that way.