Great question and a topic that's been discussed before, so you can search in the history for more ideas.
Considering my health lineage (not good!), I honestly felt compelled (or maybe I should call it more of a responsibility) to let my siblings know. I did NOT get into details (3/4), but just alerted them to the fact that I had some genetic testing and know that I have the Alzheimer's gene. NOT ONE wanted to know more, although one of my sisters actually has had the 23 and Me test for a few months, but has not submitted it. My guess is they will all begin to ask questions in due time. Anytime we are together - which is most often around holidays - they joke about their memory issues. It's sad - and even a bit maddening to me. . .
I am the baby of 5 (and in my 50s) and my siblings are all dealing with some level of health issues. . . and certainly not taking care of themselves in any meaningful way. We've already lost my oldest brother to lung cancer about 3 years ago (age 59) and one of my sisters is currently in remission for the same thing (interestingly, not the same exact cancer type). My other brother had a heart attack at 54 resulting in a quad bypass and the last sister (two years older than me) is dealing with multiple health problems. The biggest concern I've had, which lead me down this path - is that heart disease runs rampant in my family. ALL of my aunts and uncles as well as my parents are gone. And aside from my brother, we've already lost 3 other cousins. There has only been ONE cousin out of 16 of us that's made it past the ripe old age of 60!! Most of the generation above us died in their 50s and 60s. Hence, I'm on a quest to beat the odds. My biggest challenge to date has been - Environmental Toxins!!
Interestingly, my niece (27) recently completed the 23 and Me test and I happened upon that when pulling up my own data. I spoke to her about it and she freely told me she has the gene (not sure if 3/4 or 4/4). She is somewhat oblivious/ignorant to what that means - but considering she's in the process of planning a wedding, I didn't want to expand on the subject. I intend to do that sometime in the near future and when the time seems right (??).
Some good news in all of this. . . my son (also 27) has completed the 23 and Me test and is a 2/3!!!!!