The cave with the sisters is not the same as Laiken’s Study, where Sassan's ring is. Teleport to the sea side shrine and you should see a door in the side of the cliff nearby (near the north eastern most beach on the island). It is east of the wyverns' nest you could use a salve to climb up safely (or not, and take out the wyverns with a bow or magic), on the far side of the outer rock ridge.


In Broken Valley, the necromancer is in a cave to the far north of the destroyed chapel, near a teleporter shrine and a teleporter pad that asks for a password. Head north west out of the village, past where a guy called Mouse was hanging around, and keep going up following along the path by the northern mountain. The necromancer's cave is past where a small goblin camp was, at the end of where you can get heading north west.

Are you looking in the village beside the church?
When you kick the seekers out of the Black Boar inn, if you tell Louis it wasn't a big deal, things just got out of hand, you can talk to them in the barracks. They will be grateful and one will tell you about their commander's loot stash under the church, and that he often forgets to lock the hatch (coincidentally, as soon as you know this the hatch will be unlocked). Whether you loot the place or not, you can rat them out to their commander for revealing his secret.
If you get the seekers sent on a dangerous mission as punishment, you may not be able to get in (you will encounter them later fighting a troll, near Lovis' tower; if you are fast enough you can talk to them and get the hatch opened).


BTW, if you have not found the 4 parchment scrolls yet that the imp near Maxos temple can translate for you to give the password for a couple of teleporters, check out this link and/or a Broken Valley map (scrolls marked with a P ).