Ok so going off of everything, I am like 90% sure it wasn't one of the sisters that did it.
And if it was one of them it was done others out of jealousy or in reaction to a lucky pervert moment.
But most likely it was either is friend, himself or someone else outside of the family.
Also sucking on tits was probably very important to this mangas plot on progression.
Because of this detail was can assume that the culprit has large Ara Ara Tits.
Thanks for the translation
Inked Scans