AITA for not telling my boyfriend about his stepdaughter's pregnancy?

I am 19 f, Jessica is 17 f and she’s my boyfriend’s step-daughter. My boyfriend and her mom (Wife) have an open marriage policy (_polygamy ) and they are in their 40s. My boyfriend is my first ever boyfriend ever. I walked in on my boyfriend and her mom having sex and his step daughter walked in on us while having sex. She has been acting very weird since the moment she caught us. Its been some time and we all get along now. Well, the issue here is that I found out that Jessica is pregnant as I took her to her abortion appointment and I didn’t tell my boyfriend that she is pregnant. He found out and was mad at me for not telling him and her mom is mad at me too. They thought that I did that bc she caught me and my boyfriend having sex. AITA?<br><br>Edit: please stop talking about polygamy, all I meant to say is that they are in an open relationship. <br><br>Edit 2: please stop attacking her mom and my boyfriend based on the information in the post. I wrote this post to ask a question and to see how you, the readers, judge my actions not to attack them.<br><br>Edit 3: please stop commenting, its not worth it. Thank you all for your comments and I did get good points to reflect on.