If your husband is supportive of your quilting then I wouldn't worry too much about it. My husband and I take care of the money together. My husband is supportive as long as all of the bills are paid. What we do is give ourselves an allowance each payday where we can spend our allowance the way we want. You can use that money for going out to breakfast with the girls or buying the material. (Just make sure your husband gets the exact same amount.)
The amount of the allowance doesn't matter...it depends on your financial situation. Even $5 a payday will give you the feeling that you can save and buy for the things you want to get. It is important for you and your husband to decide these things together. Then when your sister-in-law wants to make a big deal about it, you won't feel like you are doing something wrong.
By the way, I agree with the others that it isn't your sis-in-laws business.