Hypothetical situation: Is is still a brother-sister controlled group if the same people own both companies, with substantially similar ownership shares in both cases, but there are no common employees in the two companies?

Yes, as long as both the 80% common and 50% identical tests for ownership are met (using 5 or fewer owners).

Let's make this a fun hypothetical. Assume both companies have employees (non common to both), and both companies maintain a 401(k) Plan. One company matches, the other doesn't. Wouldn't the controlled group rules require the nonmatching company to match, or vice-versa? That is, don't the benefits offered by both plans have to be identical? I have this exact situation going on now, where a brother-sister controlled group both set up 401(k) plans with non-similar features. This was brought to our attention just yesterday!

NO, I do not think they would both have to offer the same match. I think they just have to pass coverage.

Think of this as if it were one company with 2 locations or 2 divisions. Depending on the mix of HCEs and NHCEs, it might pass coverage. And then, of course, you would have to do the ACP test. I think the ACP test would include only the participants in the division receiving a match.