Several hours later: It seems Siren has updated their listings, including the ones I highlighted blow, claiming there is no sexual relationship between the family members involved.
This would be well and good, but it's not true. Their very own adult excerpts depict (often very poorly written) sexual scenes where both brothers will be penetrating or otherwise having sexual intercours with the same woman at the same time.
Yes, Virginia, there is a sexual relationship between family members if they have sex with the same person at the same time. This is not dependent on the definition of the word 'is', and it is not dependent on whether balls touch. Nor is there any legitimate way to claim a lack of sexual involvement between, say, two to five brothers who have spent years searching for a woman they can share between them-- even if one laughably pretends there is no sexual motivation in the setup, as soon as a sex scene happens, if more than one of them is present, there is a sexual relationship.
Siren Publishing is a company that lacks all integrity, and have no qualms with lying to their readerbase. Good show.