Well we are a family where DS got into a super-selective (he actually passed for all the super-selectives) without intensive tutoring. We did go thro' papers at home though for about six months, although it was fairly laid back as DS is not naturally an overly-keen child!
I'm afraid I'm in your DSIL's camp. It was very much my belief that if you're capable, you're capable and if you require three years of tutoring you are getting up to speed with practice rather than natural ability and essentially that is cheating.
That said, I guess tutoring does fill in all the gaps that state primary schooling and parental guidance might not....
I agree with Coralanne though with her sporting metaphor.
It is an essentially tricky one though and there is no right or wrong answer. What does worry me though, as someone from a very working-class family who did get into grammar school myself, is that this type of intensive-tutoring-to-get-in-mentality, is just so not fair to the children with natural ability from less well-off backgrounds.
Also, because we are not a well-off family (albeit a highly educated one), I think lots of (the better-off) parents at school naturally assumed that if our DS could get in then of course theirs could too.
. I think they were rather aggrieved that none of current Year 6s have got in...
I do believe though that in the areas with super-selectives, it is not as simple as throwing money at a tutor and relaxing. A child does have to have a latent natural ability. Nothing can be assumed, even with long-term expensive tutoring....