I am sorry that your sister-in-law acted so immature but in all fairness you both had different reasons to celebrate. You wanted to celebrate your reunion with your birth mom and she obviously wanted to celebrate her birthday. Though I may agree with you that she was inappropriate and should have realized that the celebration was NOT for her birthday, I am not surprised by her behavior.
Although I completely understand your frustration with her, she will soon be part of your family and has since apologized. Forgive and Forget. And for future reference-- you may want to separate other special events from the day of her birthday to avoid any drama :-)