Is her 401k account part of her estate subject to probate law since there was no will?

401(k) accounts are not probate assets unless no beneficiaries are named. When beneficiaries are named, the beneficiaries receive the funds and other people, including family members, have no claim.

A 401(k) with a proper beneficiary designation will pass outside of probate and directly to the beneficiary listed. However, it does not prevent your siblings from asserting their claims in probate court to argue why the feel entitled to a portion of that asset, it may or may not hold up in court but you cannot stop them from filing a suit against you for the money. Were there any other probate assets or assets which were held only in your sister's name without any beneficiary designation? If so, they would be entitled to a share of that under the laws of intestate succession.