Nearly all of the Cascades volcanoes are active, and most have erupted at least once during the time humans have been in the region.
Mount Rainier is NOT active and has NEVER been active during this Mt. St. Helens swarm or any time remotely recently.
The quakes at Mt. St. Helens during the swarm simply became strong enough to register at the Mt. Rainier seismographs (and the Mt. Hood ones, and basically every one in Western Washington.)
Multiple people pointed this out in replies to that horrible thread about Mt. Rainier becoming "active" started by whoever the spectacularly clueless person was that didn't understand how seismometers work but it's clear a lot of people didn't bother to read all of the replies on that thread.
Barely active enough to be measured, but active nonetheless.