There are a number of proprietary genealogy programs which would allow you to build the tree of living related persons. The downside is, you MUST back the information up, or it will be lost. (It's not uncommon to see posts in G2G of people who have lost their entire tree due to disk failures.)
Except for immediate family, I do not put living people on Wikitree, because, as others have pointed out, their profiles will likely be deleted after I'm gone (unless they precede me in death.)
There also aren't a lot of useful legitimate sources for living people, so that makes it less likely that you can create a profile with sources. US Census records aren't available after 1950. Birth certificates are generally unavailable (except through on-site local searching and paying a fee), and marriage certificates are similar. Land records are about the only accessible records for living people.
So, even though you might know the names, birth dates, spouses, and children of living relatives, you would have difficulty meeting the Wikitree standards for sourcing to create their profiles.
I think the best option for creating a family tree of the living relatives is software like RootsMagic, Family Tree Builder, Legacy, or Family Historian. But have a good backup system. (I use Dropbox.) And make sure someone else can access it if you happen to pass away.
I have created printouts of all the things someone would need to know, if I passed away, to get into my accounts, and notify the appropriate people.
If there's one thing I've learned doing genealogy, it's that people who don't plan ahead leave behind squabbling relatives that fight over their estate, sometimes for decades after they are gone.