Even without a translation, I can get the gist of what is going on. Rachel is giving Ragna an earful, and in retaliation he tugs on her twintails (I bet he's also calling her a "stupid rabbit"). Jin comes across the scene and notices how his brother is having fun tormenting the lolita vampire. Wanting attention from his elder sibling (and possibly feeling a bit of jealousy from the attention Rachel is getting), he also does up his hair in twintails and is asks Ragna to pull on them. Ragna doesn't take kindly to Jin's nonsense and they immediately clash swords.

On a separate note, should this be tagged as heterochromia? Despite the comic being in black & white, Misumimu shaded Ragna's eyes differently.

Well, I did the best translations I could do. Still need some fixing up, but the joke should be clear now.