Lost on what to do

I am a 25 year old man with a good job a 2 year old son and a sometimes loving wife. We have been together for going on 7 years married for 3. It seams that lately she is becoming more and more irresponsible. I work long hours 5 days a week 8-12 hours a day with a 30 min commute. When I get home more often then not I find a empty house with no note saying where everyone is. This wouldn't be that big of a deal, but shortly after I get home grandma (my mom) stops by and drops off our son. She then usually tells me that she stopped by around noon to grab him and my wife was still sound asleep in bed, and our son was still in his crib. Without being there to see it I used to chock this up to hear say and tell myself there is no way she would do this. Over time I started getting worried and I have tried talking to her about this. She says that she gets up with him in the mornings and grandma just seems to stop by just as he lays down for a nap. I have also talked to her mom who responded "that sounds like me with her older brothers" and starts laughing. From what I see on the weekends I am starting to believe this may actually be happening. On Saturday and Sunday no matter how much I t ry she will not wake up before 2pm. Am I right in worrying about this or should I believe my wife? As for my wife on the days that grandma drops off our son she will come home at 9pm-2am there has even been times as late as 8am smelling like alcohol and smoke. When I ask her where she has been she says "I was with (insert friends name here)" then gets defensive. I can't talk to her about this if I try she just storms out of the room and locks her self in the bedroom. I just don't know what to do...

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