Tell her yourself first. She will be hurt if you don't. My bestie tried for 6 years. I fell pregnant by accident and then had a second child before she had her first and I always told her straight up.
Her sister in law let her find out through someone else and she was so hurt. Yes it will be hard for her to hear and for you to tell
Her and she will probably be upset and won't want to know the ins and outs of your pregnancy but that's understandable.