I mean, forget consent for a second, if I can be so bold as to say so. At 14-15 you DEFINITELY know not to mess with a sibling. At 14-15, in that kind of church, you DEFINITELY know that you shouldn't be doing ANYTHING like that (not implying agreement or disagreement, just that he would definitely have known, based on the teachings he would have had since an early age).
Not understanding consent, unfortunately, is actually a higher likelihood, given that in a lot of these fundamentalist denominations, the "talk" consists primarily of "don't do it". The rules around how to do it responsibly are skipped, in order to go straight to abstinence. And, if it happens, more often than not it's "boys will be boys" and "if the slut hadn't let him..." This is, thankfully, beginning to change in some denominations, but not nearly fast enough, and certainly it hadn't changed when all this went down.
(Taking a moment to add the disclaimer that generalizing is the fastest way to be wrong, and of course there are exceptions to this allegation. But usually you don't see this crap coming out of the exceptions, sooo...)