AITA for "making fun" of a woman's weight?

IME: i am 22m, my sister in-law (33) has been gaining a lot of weight ever since she met my brother (he proposed to her), i am glad that my bro is happy, but i can't help but feel jealous because prior to meeting her i used to be much more overweight, and i thought i really fucked up my life, but then i saw her and felt that i'm not that bad, like i could live a normal life.<br><br>The background is out of the way, now to the recent events, she came to visit us last week, and when she was changing her clothes, she walked by my room, and she didn't notice me, i was in the room because i wanted to avoid her, but i didn't have enough time to leave after i saw her.<br><br>she looked at herself in the mirror, she was patting her belly and smiling, she looked happy, and for a second i thought she was pregnant, i mean the belly was big enough, but then i remembered that she is not, i felt like i've been stabbed in the back, months of hard work down the drain.<br><br>Next day she went out with me and my mom to a mall, we were walking around, and i heard her complaining about her weight, i snapped at her and told her that she can lose the weight if she wanted to, she was surprised and asked me what did i mean, i told her that i saw her smiling at her big belly, she cryed and left, my mom scolded me.<br><br>Now my brother is mad at me too, he told me that she has a disease that is making her gain a lot of weight, and she can't stop it, and that i should be more considerate, she was indeed smiling at her big belly, but not for the reasons that i thought.<br><br>AITA?<br><br>EDIT: i just wanted to say that i didn't mean to hurt her feelings, i saw myself in her situation and i snapped, i feel bad because i was in her situation, and she was trying to accept herself, i don't know, i feel like an asshole.<br><br>EDIT 2: i'm not trying to downvote anyone, but i think you are forgetting that i was in her shoes, i saw myself in her, and i snapped, i am not trying to be an asshole, and i do feel bad, but i can't control myself sometimes.