Sirris’ sign does not show up on the bridge

There's a few things controlling when you can summon someone (even with the password on).

First off, when you have password summoning on, the soul-range system is overridden, so your level 100 friend can come help your level 20 character, they will just get a normalizing damage reduction.

Second, if you are in an area where you have defeated all bosses in that area, you cannot summon anyone, regardless. You used to be able to forgoe this in Dark Souls and Dark Souls II with the Small White Sign Soapstone, but that doesn't seem to be the case in 3, as I haven't seen that item in-game. The exception to this rule is that you can do PVP summons (red sign soapstone, etc.).

Third, the summoner must have used an ember (or defeated a boss) restoring them to be a Host of Ember.

Finally, Dark Souls also enforces a "summoning cooldown" at times, limiting when you can summon your friend.

If all these conditions are met, you should be able to summon. Keep in mind it can take up to a few minutes (in my experience) for summoning signs to appear.

Also, just a sidenote: the Dark Souls netcode is sometimes a bit buggy. There are certain areas I cannot summon my friends in, but they can summon me, regardless of us having the same progress. While it's not foolproof, sometimes both the summoner and the summonee restarting Dark Souls 3 completely.

You probably still need to meet her a 2nd time in the Catacombs. She's on a narrow path on a ledge above the long bridge that you can chop down to descend into Smouldering Lake.

Basically, go to where all those skeletons spawn and look for a narrow path that goes to a higher elevation and twists to run parallel to the bridge. She's at the end of that path. If you're standing on the middle of the bridge and heading from Wolnir to where all those skeletons spawn, turn left and look up, and you should see the ledge I'm talking about.