I definitely wouldn't have coped with face-to-face or phone call. I had enough of those with random strangers, work colleagues, old and new friends, other family members and I know I couldn't have kept it together with my sister.
An email or a note would be best for me. That would give you space to say all the things you want to say about not wanting to upset her but wanting her to know from you etc. etc.
To be honest, it is still shocking to remember how raw, primal and violent my emotions could be when someone told me their great news.
It horrified me that I felt like that and I ALWAYS made sure they didn't realise how bad I really felt but it is so much harder to compose yourself and deal with sounding bright and breezy on the phone.
Oh, and I hated the face to face announcements that always came with the cocked head, sympathetic arm rubs, and well-meant but stupid comments like "I am sure it will be your turn soon" etc.
Much easier to read and re-read something in my own time and space and grieve a little for the time I won't have being pregnant at the same time as you and grieve a little for the baby I wasn't going to have.
Then I could be super-Aunt, super-friend and genuinely happy for the pregnant person.
Congratulations on your pregnancy and you sound like a very thoughtful sister.