The cousin‘s birthday is important because it helps to determine whether your wife is an eligible or not an eligible designated beneficiary. If your wife is more than 10 years younger than the original account owner, then your wife is not an eligible beneficiary. That means she must follow the 10 year rule rather than the RMD rule. She must spend the account to zero and close it within 10 years of inheriting it, but does not have any annual requirement other than that.
If your wife is less than 10 years younger than the cousin, then your wife is an eligible designated beneficiary and she may withdraw on any schedule and keep the account longer than 10 years if she wants to, but she must withdraw at least her RMD amount each year.
