My stepsons mother was arrested for dui/with a minor in
the car... the minor was her daughter (not my husbands
child) we share custody 47% to 53% Mom.... what are our
legal rights in this - we would like to go to court and
get primary residence in light of this...
You probably shouldn't do anything until she gets convicted. If she's convicted, and if the report shows that she was DUI with a minor in the car, I think you have a reasonable change of circumstance argument.
I think you'd have a decent shot to argue that Mom is willing to drive drunk with her own child in the car, and that you're concerned about the welfare of your stepson.
Just depends upon any other skeleton's in mom's past, the judge, and how good you make your case. If she has prior DUI or drug offenses, that would raise your odds considerably.
But if mom has a clean record and says, "I was so sick, I was drinking Dayquil all day long, and I just didn't think about its alcohol content", then your case is weakened considerably.
This website gives common sense advice that is not intended to act as legal guidance nor psychological guidance. The author is neither an attorney nor licensed psychologist. For specific legal guidance or specific psychological guidance, consult with a licensed professional.