Unfortunately, unless the bio parents sign over and let you adopt the children then there is no other guarenteed way of keeping the children. Bio parents have a great deal of clout with the courts. If you had guardianship for 5 years the bio mon could still come back and get the children via a court order.
Now, if you are given full custody by a court then you have a little more to fight with, but a judge on a good day would change physical custody in a heartbeat if the bio mom showed that she had changed and wanted her children back.
Obviously, things change over time. If you had custody for 12 years it would be unlikely that she could get custody back.. but visitation is a cert anytime she wants it.