My sister in law has had her sewer constantley backed up for over a year and some. The smell is horrible. She has two small children and I was wondering what health hazards it may have on them.

A build up of sewer gas is dangerous to your health and it can explode if concentrated. What has she done so far to get the drain rodded? City sewer or septic tank?

Is this through a floor drain? E. coli is a big possibility, mold and mildew, insects and vermin. Get the sewer opened and then pour a bleach solution on the floor and scrub away.