I saw a TG meta (forget whose it was) about Hinami feeling left out because of Kaneki and Touka moving on and moving closer without her, wherein this statement was backed by a panel from the manga of Hinami looking at Kaneki and Touka’s backs.
Meanwhile, in Japan there’s a saying that literally translates to “Children grow up looking at their parents’ backs” which means that children look up to/emulate their parents (therefore parents should make sure to do well by them). Seeing your parents backs means you’re too close to them, too protected, not too distant.
Hinami looking at Kaneki and Touka’s backs very likely was meant as her feeling reassured and safe with them (in addition to making sure the reader realizes she thinks of them more as her relatives).
It’s interesting how differences between cultures affect readings.