Okay, just imagine those two princesses :
being best friends.
Imagine them arguing for hours, trying to decide wich one is the best between swords and guns. (Lis would be for swords and Sinon for guns) Imagine them in Lis shop talking for hours while Lis fixe Sinon’s bow. Imagine Lis telling Sinon what happened in SAO in details and Sinon telling Lis what happened in GGO in details. Imagine them teasing each other about Kirito. Imagine them teasing Kirito together. Imagine Lis making bad jokes on purpose to make Sinon laugh when she’s depressed. Imagine Sinon in her bedroom screaming and making a panic attack and Lis, who was in the kitchen, running into the bedroom and hugging Sinon to calm her down. Imagine Lis helping Sinon to, little by little, overcome her fear. Imagine Sinon being there for Lis when she’s depressed, holding her hand, and telling her sweet stuff. Imagine them being confidents. Imagine them being best friends.