What everyone else says.
MIL's rship with her daughter is none of your business.
When we first moved in together they offered us no financial help at all ..well that's tough luck I'm afraid- some parents help, some can't- and some just don't. It isn't something you should just expect. A lot of parents think if their Son/Daughter wants to move in with their OH, they should be financially able to do so? If you already had 4 DC's before meeting him, I presume you had a family home anyway 
If anyone is going to say anything, it has to be her Son, not you which will still fall on deaf ears
I don't think the major issue is your MIL actually, it's the fact you expect the same help and financial support that her Daughter gets. And if it hasn't happened before now, it isn't going to change.
Inequality can be massively annoying, my Ex MIL used to favour my XH's daughter over our DS. She never had a girl of her own, family full of boys, so she saw my Son as old hat. She actually went NC with her GS when i divorced her vile DS. And that was her loss
We are thankful, not bitter.
You can't force someone to feel something they don't. And as for you being annoyed about lack of 'financial help', it makes you sound a bit over entitled.
Babysitting and support are not guaranteed when you have a family. It's great if you do, but you can't expect it- or be pissed off of you don't get it.
She's made it pretty clear she doesn't want to babysit 6 children, even if they are her DGC. I can empathise a bit with her, my DDs have their friends round a lot, and it's chaos, she may find it very daunting.