I don't know where you live but most places in the US will commit patients if they have ideation/intent, a plan and a means (ie gun, rebreather, etc).
So, I guess it would depend on how much your family member knows. Planning to order means is just planning - good healthcare and mental healthcare providers will not overreact to planning (bc detainment increases risk of ctb significantly). That said, many providers do overreact to planning and will hospitalized patients for simply planning.
Are you concerned about potentially being hospitalized? Do you have additional supports in place? Do you think you can handle waiting to order your backup until you do whatever it is you're doing? Are you open to telling your family member that you're thinking about doing this instead of that you are?
I hope your family member is open and accepting of whatever you decide to share with them.