I have done this mission several times for the fun of it because I dont plan on ever buying the buckaneers set of armors.
After getting the mission you need to approach the undead island just out in the sea from the main shore camp. There are two roamers on the north side, two on the south side and two static mobs in the middle with one of those being inside the ruins. There are also two swimming roamers west of the island but just in the water there.
Your 4 girls can spawn in any of these locations and they can spawn more than one sister of the same type at the same time. Its totally random, but usually you will get one to three on a spawn. They all spawn at 7:00pm game time and all despawn at daybreak. Their PH can be a zombie or a skeleton type mob which are around lvl 55-60 or so.
IF you clear the two roamers on either side of the island you can pull a static mob without getting adds but either way you can also fight them under water away from other roamers.
They will respawn if its still night time and its the same random respawn if you get the named or PH's. If you dont get all four different ones in an evening, then you will have to kill sharks or something during the day as you wait for 7:00 pm again.