"Is there a way to mark/indicate direct relatives?"
No such option from within either ATM or FTM. As has been discussed here regularly, you're rather on your own even if you elect to use the traditional all caps methodology (tho FTM may have that as an option...I'm not sure on that).
My preference is to utilize ansi characters (I hate all caps and ANSI is now recognized by FTM)...eg. ♥ John Paul Jones. They are unobtrusive and, I think, rather innovative.
Your mileage may, of course, vary as will that of others who might contribute here...but you're kinda on your own on this one and I wouldn't expect ACOM to rock the genealogical boat in this regard anytime soon.