“I never know” is in the simple present tense, and you use it to talk about something usual or habitual. For example, you can say: “I never know how to tie my shoelaces properly.” This means that you didn’t know how to do it and still don’t know now.
Another example: “The bus comes so irregularly, I never know when to expect it.” This means that because the bus is so irregular, you cannot guess when it will come, and this was so in the past as well.
“I never knew” is in the simple past tense. We usually use it to say emphatically what we didn’t know until now. For instance, we can say: “I didn’t know you were sisters.”, but to give emphasis to your statement, you can say: “I never knew you were sisters!”