Hi. I never received both of my stimulus checks. It said both went to my sisters bank account that I used but never saw it how do I find out what happened to my stimulus

Most banks will reject payments to an account that does not belong to the taxpayer.  If the bank didn't deposit the funds in your sister's account, they would have been returned to the IRS.  In most cases, a returned payment will result in an updated status in the IRS lookup tool.  Returned payments that did not result in a check being issued can be claimed by the taxpayer when filing the 2020 tax return.  

The IRS made a statement on January 5, 2021 about the current Get My Payment statuses and what they mean.  You can read the full text of the IRS statement here: IRS Statement About Second Economic Impact Payment. Below is a pertinent excerpt:

I used my sister’s bank account because I didn’t have one at the timing. Both said it went to that account but she has not seen them. She even provided me all of her bank statements as proof

IRS updated Get My Payment (GMP) for individuals who are receiving the second Economic Impact Payment on January 5, 2021. If you checked GMP on or after January 5 then:

• If GMP reflects a direct deposit date and partial account information, then your payment is deposited there.
• If GMP reflects a date your payment was mailed, it may take up to 3 – 4 weeks for you to receive the payment. Watch your mail carefully for a check or debit card. (See the FAQ for EIP Card)
• If GMP shows "Payment Status #2 – Not Available," then you will not receive a second Economic Impact Payment and instead, you need to claim the Recovery Rebate Credit on your 2020 Tax Return.

Because of the speed at which the law required the IRS to issue the second round of Economic Impact Payments, some payments may have been sent to an account that may be closed or, is or no longer active, or unfamiliar. By law, the financial institution must return the payment to the IRS; they cannot hold and issue the payment to an individual when the account is no longer active. If Get My Payment shows "Payment Status #2 – Not Available" you will not receive a second EIP.

 

The IRS advises people that if they don't receive their Economic Impact Payment, they should file their 2020 tax return electronically and claim the Recovery Rebate Credit on their tax return to get their payment and any refund as quickly as possible.