Planning and hosting a kid's birthday party is stressful enough, but this woman's entitled sister-in-law made it so much worse. This OP shared her story with the internet, since she was seeking helpful opinions on whether or not she was the jerk in this situation. As the OP, u/haley××2 wrote, she was planning a birthday party for her kiddo, age 8, and inviting a bunch of people over. The OP invited her sister in law and niece, age 6, to the party as well. Since her niece has several allergies,…