Is it possible for you or someone else to usurp an Emperor title? I married a woman with a weak claim on the HRE to my son (so she is now in my realm as my sons vassal) and killed off kaisers until an 8 year old took the throne, but I still can't press the weak claim even though the HRE is in a regency. Are the conditions for pressing an empire claim much more restrictive?
As Affine points out, to press a woman's claim on a title, the title has to be agnatico-cognatic or absolute cognatic.
You can't see the laws overview for any kingdom or empire you don't personally control a duchy in. But you can see the succession type for any title by hovering over the shield. The message that pops up after a second will tell you the current succession type and the line of succession for the first three who stand to inherit.
Also, Studoku's excellent summary of claim pressing criteria in this answer are worth keeping in mind.
Edit: Your question text asks about 'usurping' an Empire title, which is different from pressing a claim. You can usurp any title by controlling 51% of the de-jure counties, empires included. For the HRE, that includes all the counties of the Bavaria, Frisia, Germany, Lotharingia Kingdoms, at which point the former Holy Roman Emperor would fall back on their next highest Kingdom or Duchy title they own. If it's a duchy, this could really fracture the realm by creating many independent dukes, most of whom you could declare war on immediately with de-jure casus belli's.