This is so dumb, but you wouldn't believe the impact it has had on my family. 

I'm 19, I have twin girls who are now three years old. I'm a university student living with my boyfriend of four years in a small apartment, with two small rescue dogs. We don't have a whole lot of money, but we can live comfortably as long as we don't waste our money. Because of this, when me and my partner decided to get a dog last year, we chose adoption over buying a puppy (there were a few reasons along with money, such as wanting to rehome an unfortunate dog and also, we wanted to have a slightly older dog that was more adjusted to kids). We went to the shelter and found two beautiful boston terrier girls. They are both two years old and sisters, like my daughters were at the time, they had been around kids in their first home and really seemed perfect, so we got both of them. They are amazing and we love them so much, the kids love them and the dogs are really well behaved. 

Because we have a two bedroom apartment and not a whole lot of space/money, my boyfriend and I donated and sold the baby clothes as the girls grew out of them, keeping only the sentimental pieces. However, we fell a little bit behind somewhere and ended up sorting through quite a lot of 0-6 month clothes that we left at the back of a closet. Now, my sister in law has recently found out she's pregnant. Her and her husband are really well off and had a son a few years ago who was dressed head to toe in designer clothes from birth. I didn't think they'd struggle for clothes. My boyfriend suggested we could give the clothes to his sister when she found out she's pregnant at a family meal, where my SIL was in attendance, but everyone sort of laughed it off so I thought nothing more about it. 

Fast forward a month or so, we finally get round to sorting the baby clothes. My husband and I had bought the dogs some matching sweaters setting us back £20, so I had the grand idea to start cutting and sewing the clothes into dog sweaters and onesies for them to sleep in through the winter. My boyfriend thought this was a great idea, the girls loved seeing the dogs in their old clothes and it was super cute. I posted pics of them on instagram, and basically got a snarky text from my sister in law about how she'd have to be buying a new wardrobe then. I took it as a joke, and sent her a few more pictures of the dogs thinking she was being silly, and she kicked off. MIL started kicking off too. Next thing I know, my SIL is posting on Facebook marketplace asking for cheap/free baby clothes. 

So, AITA for assuming my SIL would buy her own baby clothes when she didn't seem too pleased to have mine/dressed her first baby fully designer? As far as I know they haven't had a change in income, but am I wrong for assuming?