How/when to share sad pregnancy prognosis with my sister/soon to be mom?

Hi.... CW, mentions lethal prognosis for my baby

I am 25wks along with my second baby...at the anatomy scan a few weeks ago we received horrible news that she has severe brain, spine, limb and facial profile abnormalities that are "not compatible with life". We're told she could be stillborn, pass shortly after birth, or best case survive days/weeks

My sister is due any day now with her first baby and has asked multiple times how we're doing/how my pregnancy is going etc. I've managed to divert the conversation to her upcoming birth. We live on separate sides of the country so communication is all done thru text/calls which helps

At some point im going to need to update her though and im trying to decide when best that would be? I feel like leading up to her due date is too heavy, esp this being her first. But early newborn days would be sad/could taint things as well. Im trying to hold off until her baby is like a month old? But avoiding this conversation for like 5wks also seems a bit daunting, esp since I have a lot of family members we're slowly starting to share with. Any advice??