parents partying every weekend?

Ok so my husband and his sisters get together at his moms house every Saturday  to have dinner. And 90% of the time they usually drink. Which is fine, is rather my husband drink at his moms house then out at a bar. The problem is they usually drink until the liquor/beer is gone. Which usually goes until 4-5 am. Me n the kids are ready to go by 1-2. But since we brought 1 car we have to wait for him. His sisters kids just sleep on the couch or floor until their parents are ready to leave. Then cuz we get home so late he sleeps all day Sunday while I'm up taking care of the kids. So we always end up fighting. Am I over reacting? Or is drinking til 4am every weekend ridiculous n unacceptable behavior for a parent of a 7 yr old n 3 month old? Let me add he works second shift so me n the kids don't see him during the week. The weekend is supposed to be our family time...