Profit Sharing/401(k) issue

The central issue is whether or not a family member can be excluded entirely from the plan (0 deferral, 0 SH 0 PS)?

In general, you can exclude anyone from participation in a plan, as long as you have a definitely determinable way of doing so. You could exclude them by name; for example the plan could say that "The definition of Eligible Employee does not contain Joe Smith." There are other ways to do it as well.

If the employees being excluded are all HCEs (and if they are the spouse/child/parent/grandparent of a 5% owner, then they are HCEs) then this exclusion should not cause any issues with your coverage test.

How excluded classifications of employees will interact with the upcoming LTPT rules is still TBD.

You didn't present enough information to say with any certainty. In general, you can exclude Highly Compensated Employees. If the family members are HCE's (and I'm assuming you are asking this question re HCE's by ownership or attributed ownership under IRC 318) then you should generally be able to exclude them, but "family members" can cover a lot of ground. No attribution between siblings, just as an example. You need to look at the specific ownership and relationships.

HCE, NHCE, family member... all should not be an issue if the individual is excluded from participating by name as long as the plan passes coverage.  If the plan uses rate  group testing, then the individual will be in the test as non-excludable, and also the exclusion cannot be used to pass the reasonable classification part of the average benefits test.  Short version, know where the mines are buried so they don't blow up compliance.