Besides being awful in about 17 different ways?
That’s because the mere act of volunteering for that role would say to or about your sister-in-law, “I don’t think you’re up to this.” And who says she’s not up to it just because she is different from her daughter, and her views are different from yours?
If you care about your niece, then care about your whole niece, not just her early bloom. Talk to her about all kinds of things. Even better, listen to her about all kinds of things. Your interest is suspiciously close to being about how groovy you are, vs. what your sister-in-law and niece actually need.
Soon after, her business did a 180-degree turn and has made phenomenal profits for the last three years. She now is beyond flush with cash.
We now have financial pressures and I would like to ask for the money back. Because my mother-in-law has given my wife extra money and gifts for Christmas and birthdays for the last three years, my wife believes her mother owes us nothing.
This has led to several arguments and apparently I’m a jerk for not recognizing that my mother-in-law has paid us back already through gifts.
Your thinking becomes flawed when you try to use that math to solve a problem that’s personal, not business. Your wife doesn’t want to ask her mom for the money.
Your objection is legitimate but it’s her relationship with her mom that’s on the line, so I encourage you both to acknowledge openly the validity of each other’s position on this. Then, accept that your wife’s bond with her mom trumps money.