"Periapsis" does not seem to be appropriate as it describes a specific point in a single body's orbit.
While we start by thinking of a central immovable body and tiny essentially massless satellites in orbit around it, we know in our hearts that this is wrong and everybody moves in an orbital system. Even the Sun moves around the solar system barycenter (dancing with Jupiter, Saturn and Neptune mostly) so much that the barycenter is outside of the Sun roughly half the time.
We can look at our Earth-Moon system where they circle around a spot roughly 2/3 of the way out from Earth's center to its surface, or better yet, the Pluto-Charon system.
Wikipedia's Charon gives the following:
||19591.4 km (planetocentric)
17181.0 km (barycentric)
The difference of ~2400 km between the barycenter and planet-centered semimajor axis values is more than double Pluto's mean radius of only 1,188.3±0.8 km meaning that the point they orbit is in empty space.
There's nothin wrong with using periapsis and apoapsis in both the limits of a low-mass orbiter and a central body and of two more equally-massed objects.
It's simply important to note how you're using them.