one thought not mentioned -- with a busy household that includes several young students coming and going for holidays, etc... there would have been probably have been fairly constant contagion in the Austen household. Putting the babies out to be cared for by a trusted servant in a more secluded, quieter home nearby could have been an important protection against disease. Remember that newborn infants are protected for a few months by the mothers immunities, but that wears off and the baby becomes susceptible... farming out a 3 month old for a year or two gives the child time to develop a more robust resistance...