I have been trying to fix my credit. Between my X

Ok you have several issues going on here. First, you are NOT liable for anything your ex did or your sister did in your name as this is identity theft...however....you will need to file a report with the police and FTC.

Now for the others it is a simply matter of sending out debt validation letters referencing FDCPA, FCRA and FACTA. So they say you have been paying on an account for 4 years? Fine...make them PROVE it. You want to see a signed contract/agreement, the last statement, proof they are the correct entity to be collecting and proof of these alleged payments.

Now before you send this letter you also want to scrutinize your credit reports. 9 times out of 10 collection accounts are reported inaccurately such as listing the account type as ANYTHING other than collection account (like factoring company account, installment, etc) you see, once the debt gets to a collection agency it is a collection....the original creditor may list an account as installment, but the collection agency may not. They also often reports terms and a past due amount. Unless you made arrangements with the CA there are no terms and there can't be a past due amount if you never entered into any type of agreement with them in the first place. Plus, CAs are notorious for adding in bogus interest and fees...well they have to PROVE they can add these charges with the agreement/contract you signed. So if the amount they are collecting seems off, you can demand proof of that as well.

You can post the info of some of your credit report and we can help you craft a letter.

There are a ton of debt validation letters out there but you have to do your homework or ask for an opinion of the letter here, because some letters are just crud and would get laughed at and some are very good, BUT all letters should be personalized to each particular person and account and not look like a form letter.