Each Family Plan can only have a maximum of 5 lines. What you can do is start a new plan under your account. This could be an Individiual Plan for just the new line, or a new Family Share Plan with one or more of your current lines. If you did this, you'd have to play around with the numbers and analyze the usage on each phone to find the cheapest way to break things down.
Your Mom and Dad could also move to their own 65+ plan, but only if one of them is 65+ and the account owner. They would have to do an Assumption of Liability to take over ownership of the two lines from you. Just call in with both you and the new owner present, or visit a store together to get that done. This won't impact the contract dates on any of the lines involved.
Does that mean you can get company discount( like 20%) at two different plans within the same account. I have a question about how this discount work. I am goanna to move my line from my family plan to a new individual plan, but my sister has 22% discount and she is the primary line. If I only move from this family plan but don't move from her account, will I still get this 22% discount? Thank you