@SeekingSolace the girl bought a massage tool that is two rollers attached to a handle, which you can rub on your face your massage it.
The brother took the massage tool, placed it at his crotch, and used the shape of two circular rollers attached to a handle to declare "hey look it's my dick".
Taking offense to her massage tool being intimated to be her brother's gentials in such a crude way, along with actually putting the tool on his crotch, caused the girl to be aggrieved, and she therefor kicked her brother in the stomach hard enough to draw blood.
Context: The girl is using a face roller, which is this holistic tool used to massage for skin benefits. There are several types, and this particular one has two balls shaped to fit around the curves of the face.
Truly a harrowing representation of the struggle to keep individuality in a fast-paced world. This belongs in a museum. At least, I hope there's a six season anime made out of this.