The character was made and animated by Aleksandr Ladosha.
Have a look at Big Sister, an absolutely incredible character animation with an Arcane-inspired hand-drawn-like art style, created by 3D Character Artist Aleksandr Ladosha.
According to the creator, the project itself started at the end of 2020 but would only be completed after the release of the Arcane TV series, which inspired the artist and gave some ideas regarding the character's final looks. The project's description states that the artist utilized Maya, ZBrush, 3DCoat, and Marmoset Toolbag to create and animate the character.