You're totally good to continue sellen's story after taking down radahn, actually it's a necessary part of moving her quest forward. The important thing to understand is that beating radahn opens up some really interesting developments in her storyline.

First thing you'll want to make your way back to sorceress sellen in the waypoint ruins cellar, where she'll tell you about her body being trapped somewhere else. From there, you need to track down her chained-up form in the witchbane ruins down in the weeping peninsula, and grab what's called her primal glintstone. This is where things get interesting because you'll need to find a secret area near ranni's rise in the three sisters region, there's this hidden basement with an illusory floor that's pretty easy to miss.

Once you've got all that sorted, you can transfer sellen's consciousness, and eventually you'll run into witch-hunter jerren who's got his own plans. The whole thing comes to a head in the grand library at raya lucaria, where you'll see two summon signs and need to make a choice about whose side you're on. Just make sure you've already picked up comet azur from mt gelmir and stars of ruin from sellia hideaway, since you'll need those to see everything through.

Tip: when you're looking for that hidden basement in three sisters, keep an eye out for a suspicious-looking floor - might save you some time wandering around!