Oceanids are the children of Oceanus.
All nereids are nymphs, but not all nymphs are nereids. The difference isn’t just, “water nymph”. Nereids are the children of Oceanids.
(Naiads are basically the same except fresh water. Naiads are the daughters of the Potamoi, who are children of Oceanus :D)
Penelope is a nereid. Amphitrite’s grandfather is Oceanus, so she’s also a nereid. Penelope and Amphitrite are cousins.
Do with that what you will.
Edit: Ask and you shall receive. Screenshots from my reference folder:
Cousins.