How to Avoid Hurt Feelings?

Colby
My sister is 3 years older than me & recently divorced. They were trying for a baby with no luck before they split. My husband & I have been trying since February. She knows we are trying but got upset with the topic (her dr. told her it was going to be next to impossible for her to have a baby ever.) If I get pregnant how do I tell her? On one hand if I tell her she may go into a downward spiral I don't know if she can recover from (she's always depressed) but if I don't tell her she'll be livid I didn't tell her first.😫 How do I tell her gently and at what time?