“The earliest battles were internecine and costly, piles of dead crowding battle-lines that barely moved.”
Note: Internecine comes from internecinus, the Latin word meaning ‘very deadly, murderous, destructive’, a definition that is still common. [EtymOnline.com]
Modern usage, however, often connotes internal conflict, as in, “Internecine power struggles within the regime led to its eventual downfall.”
Taking these variations into account, it’s best for the internal conflict to be a little… murderous.
“The internecine sibling rivalry took an unexpected turn during a church outing to the axe-throwing venue.”