If one parent has been charged with a DWI what are the chances that they will lose more custody to the other parent? The child was not with the parent that got the DWI as it was the other parents week-end.

Evidence of their excessive use of alcohol will certainly be relevant in a custody case. A DWI can certainly be considered, but it is up to the judge about whether or not that alone (without more) represents a substantial change in cirumstances warranting a modification of custody. If the child was with him (thankfully he wasn’t), then this would very likely merit such a change.