Ouch, to be honest I don't even know how I would handle that situation. I think your SIL really had no business telling you how to spend your money. However, I also know that family doesn't always mind their business when they should, and the simple fact that they ARE family makes it even more difficult. If she does it again I would let her know you care about her and respect her opinion, but how you spend your money is your business and yours alone, and ask her to kindly keep those comments to herself. I have had a hard time in life learning how to approach these things without getting into a big fight, so I really feel for you!
If it makes you feel better, I am a SAHM of two children with a third on the way. Needless to say we live on one income, with the exeption of what I earn several days a week babysitting. Seems everyone has an opinion on what we do and how we do it. Either I don't work hard enough because I don't have a job, or I have no life because I'm home with the kids, or we somehow don't do enough for our kids because we don't spend hundreds of dollars on new clothes for them each year, or my kids aren't independent enough because I'm home so much...ugh. I'll tell you what, I don't get my hair done, my nails done, we don't drive expensive cars nor do we wear expensive clothes...God forbid, while I do spend a portion of money each year on new clothes, most of my kids play clothes come from thrift shops and yard sales. The one thing I do splurge on is my quilting. I buy the best fabric I can afford because I LIKE it, and I do it without shame. Our bills are always paid, though it may not be in the order or manner that others would prefer, my kids don't want for anything, and I think if I didn't get to splurge once or twice a month on fabric I might go nutty! It's nobody's business but my own and my Husband's and luckily he has his own hobby so he understands.
Some people don't get it but at the end of the day it's YOUR life and you've got nobody to please but yourself. I guess I'm getting to this point in my life where I'm just over other people and all their opinions. To be frank I think people need to learn how to just SHUT UP. LOL* You know how they say patience is a virtue? Well so is SILENCE. LOL*