My fiancee's been divirced for 5 years and now his ex wife wants ac opy of his life insurance policy naming his kids as the revocable benificary, does she have the rights to it if his sister is the benificary?

If he was ordered to provide this in the original court order, yes she would be in the right to be sure he has followed the court order,

1. was there an original order to provide this, ( it is common)

2. if there was, does he have the policy, if he does not, he can be taken back to court and if he does not provide it, be found in contempt of the court order

As pointed out, it depends on the divorce order. If he was ordered to make the children beneficiary, then she has a right to see it. If there is nothing ordered that he provide for the children using life insurance and he has changed the beneficiary, then just tell her he's changed the beneficiary.

This has nothing to do with statute of limitations. It's a matter of the divirce decree.