If this were me and I had a relative/MIL that was doing what your MIL is doing:
1) I would be major irked.
Why?
1) Because, it is for the parents of the new baby, themselves, to decide when and IF, they even want visitors, in their home, when their baby is born and home.
It is NOT... for others to decide. It is not for others to decide, when and if to visit.
So the bottom line to me is:
1) You NEED to ASK your BIL and SIL what THEIR personal plans are... once they have their baby and if they even want visitors IN their home. Now or right away or later.
That is me. That is how I view it.
If relatives/in-laws made their OWN plans (without asking me first and came to visit anyway despite my preferences), to come and visit me/my husband in our home... when we had our 1st baby... I would be livid.
Why?
1) because, it is rude, to just drop in and invite oneself to a person's home/hospital room, without being invited by the parents themselves... to see their 1st baby.
AND, they also have... other relatives. Meaning, their immediate family and non- In-Laws, too. And they all may have different plans.
And it is not up to MIL, to decide any of this.
And you have financial concerns and cannot just spend spend spend like they can.
So, you do not have to go.
You have a child too.
You can all "Skype" each other.
Online.
It is free.
That way you can actually see them and talk with them.
Personally, when I had my 1st child... I DID NOT WANT VISITORS AT ALL. Not at the hospital and not at home. Nor did my Husband. I didn't even answer the phone for the first couple of months at least.
"I" chose when, visitors were then invited.
I would really have been pissed, if relatives/people/friends, just showed up on my doorstep... To "visit baby" and made their own visiting plans.
It is not up to them or their choice.