Hi!

On the Etymonline.com I've found supposed etymology of word "barter".

barter (v.) - mid-15c., apparently from Old French barater "to barter, cheat, deceive, haggle" (also, "to have sexual intercourse"), 12c., of uncertain origin, perhaps from a Celtic language (cf. Irish brath "treachery"). Connection between "trading" and "cheating" exists in several languages.

I wonder if there is any relationship between Celtic words for "treachery" and "brother", "fraternize" or similiar terms, and if so, how did it happen?

My guess.
Franks were Germanic tribes. They want to subdue Celts. System of exchange by which goods or services are directly exchanged for other goods or services without using money is disadvantageous for authority, because authority wants to control, authority wants to meddle in peoples affairs. Therefore, the exchange of goods between the brothers has been stigmatized as a cheating.

But this is only a guess. I don't know any Celtic language. What do you think?

Seems like a pretty big reach to me.

Redwolf