How tight is too tight?

Anyone else had trouble breeding son back to mama? Made a cross like this a few years ago. Pups met all expectations in regard to hunting, but had one with a terrible immune system that got sick and died at about 15 months and two others in the litter that were just poor-doers (fed well and had wormer running out of their ears but always looked underweight and had rough hair coats). I've been told that a mother-son cross is harder on the gene pool than a father-daughter cross, aka more likely to have unwanted effects.