Hallo COV, have just caught up on your thread and see that it involves far more than just an anniversary date!
I agree with the other's advice. Yes, families help each other out, but your MIL seems to be asking far more than the norm and it's interesting that it's coming from her and not your SIL. Is she still unaware of this plan?
My parents helped me buy my first flat when I was 30 and then, due to unforseen circs, they offered a small loan to help us buy our present house - but it was THEIR money and THEIR choice. I would do the same for my dd if necessary - and if I could afford it. And having had this help, all further expenses and payments are my responsibility alone. However, I would never have dreamed of asking for anything from my sister, even though she and BIL are better off in bigger house, better area etc. For your MIL to suggest this is outrageous, and particularly to suggest a regular payment, an outright gift rather than a loan.
It seems that she's a very controlling woman, who can't face the fact that she has adult children who should (and in your dh's case do,) stand on their own feet. Your SIL is a graduate of one of the best unis in the country and should be forced to grow up. She obviously has a personality that has fed on her mother's possessiveness. Of course, your MIL will blame you if her ds stands up to her - because he's still her little boy who should be obeying HER whims - not a man with a wife and children and responsibilities of his own. OOh, it makes me want to spit - but I think you will have to risk taking this blame or you will resent him giving in to this power-mad 'lady' - as her own poor husband has already done all their married life, no doubt, for a quiet life! Time your husband told his mother that he's got his own family to look after - and if SIL can't afford to pay her mortgage, she'd better downgrade her property ambitions. Can't she find a one-bed flat as a starter home, just as the rest of us have? (I suppose she needs a 2-bed flat so she'll always have a spare room for Mother .
Have you had your talk with dh yet? And stand firm for your weekend, too. Good luck.
Me, one of the best things about my divorce was knowing I need never see my MIL again! Dp's mother is so much nicer.