Reasons why you shouldn't

let your mother in law move in:
 
1) Having to make a decision between what's right and what your MIL says
2) Never give up the status of Alpha Male

That's funny.  "Let" your M-I-L move in...  As if your opinion would be given any consideration at all...

PWF (and yet, sad at the same time)

Wait till she asks you (in front of your family) if you're still wasting time on that silly music thing.  It's often followed by watching her turn to her daughter and say, "You poor dear.  He's not right in the head, is he?"  Depending on which one of you your wife finds more irritating at the moment, she may side with either one.

You'll have to up your game.  Her first order of business will, of course, be to move you to the patio or back alley so she can establish "her new home".  Don't take this lying down.  Instead, encourage her to spend increasingly more and more time with your wife. 

There's a reason your wife moved out of that woman's house and settled for with you.  The more she spends Quality time with her daughter, the more that vein in your wife's forehead will start to throb.  Yeah.  That one.

The atmospheric pressure inside the house will rise to mythic levels.  Keep calm, and pretend you don't know what's going on.  Yeah.  Just like that.  Give it time to build, fester, and bubble to the surface.  You smell that?  No, the other thing.  No, not that one either.  That one.  That's frustration.  Frustration and disappointment.

For once, however (and this may be the only time, so savor it), this is not directed at you.  Don't celebrate yet.  The situation is volatile and very dangerous.  Things could turn on you in a hearbeat.  Let nature take its course.  Before long, your M-I-L will cross one line too many and your wife will put her down like a rabid ferret.

When that happens, you're not quite safe yet.  Comments like, "I told you that old hag was trouble!", "Ding! Dong! The Witch is Dead!", or "When you're really mad, you look exactly like your mother!" may backfire unexpectadly.  For now, the best course of action is to quietly side with your wife.  You want to be out of her field of fire, without drawing undue attention to yourself.

After a suitable cooling off period (timing will vary), it may become safe to return indoors on a semi-regular basis.

Hope that helps!

We hope to build a mother in law apartment here.

I guess I'm lucky... me and mom have worked out our very small differences and we are enjoying just enjoying now.

My wife loves spending time with her mom... it makes her happy.

When my wife is happy I'm happy.

:-)