Susan,
My son (8) is tongue tied and hypotonic, so he would fall off the breast
constantly.  He has never yet had a cavity, but his teeth are awfully crowded
from the failure of his palate to expand.
        I think the 5% figure that your sis-in-laws dentist quoted is really
high.  In my 7 years working with bf women I have only known 3 dyads with
dental caries/decaying teeth.  One was attributed to a febrile illness,
another was familial weak enamel, the third is a mystery.  I agree with LLL
that some kids develop caries in spite of breastfeeding, not because of it.
        Another factor in the debate is the issue of iron supplementation.
Lactoferrin plays a role in cavity prevention, exogenous iron saturates it,
allowing cavity producing bacteria to grow in the mouth.  The mania for
putting kids on iron fortified cereals as their incisors emerge could
conceivably contribute to tooth decay.  I usually advise moms of healthy full
term babies to have the ped spin a 'crit if concerned about iron status, and
if all is well, offer the baby a few shreds of meat at the dinner table as one
of the first solids.  I am confident that the natural iron is more absorbable
and there is less of an 'overdose' to overwhelm the lactoferrin.  Of course, I
advocate solids at developmental readiness- when the children can sit, pick up
the food, and shove it in their own mouths!  Just my humble opinion.
Catherine Watson Genna, IBCLC