Maybe this article is helpful. http://www.brightscope.com/financial-planning/advice/article/24529/10-Ways-To-Find-An-Old-401K-Plan/
Not enough information really. What kind of account was it? How is it currently titled? In general, it would be determined by the beneficiary designations on the account. Contact the bank in question to see if they will tell you the information. Ask the executor, if there is one, for that information.
I would agree with Jason on this one. Not enough information. I would call the institution where the retirement account is held. You need to determine who the beneficiaries are on the account. Basically, the assets will where the account has them directed to go.
I agree with the above...this is a tough one with more questions than answers. Your dad passed away previously and I would assume that the beneficiary of the account was your mom. At that point, she should have taken over the retirement account but in her name. She then would designate her own beneficiaries on the account if something were to happen to her. It doesn't seem like that happened. This could get complicated because when your dad passed away, the retirement account becomes your mom's account. Now that your mom has passed, it's part of her estate and should go to her heirs. Have the executor get in touch with the company holding the retirement account and you may need to get a lawyer involved if things get sticky. Good luck.