I have full custody of my kids an my sister got granted guardianship.

[justneedsanswer]

Can she keep me from seeing them?

 

[army judge]

Can she keep me from seeing them?

If things are as you say, YES, she's stands in the place of a parent.
If someone has guardianship, as in LEGAL GUARDIANSHIP of your kid, you have nothing anymore.
They are no longer your legal responsibility, but they'll always be someone you can love.
Maybe when they become adults, you can reunite?
The person has full control of the children, as their LEGAL GUARDIAN.
I suggest you speak with a lawyer, so you can come to learn just where you stand.
In the meantime, try to recall how this ended the way it did.
Then, work on things to see if you can fix what's broken.