I'm a US green card holder. I lent my US earned after tax money to relatives in a foreign county (parents and brothers, sisters) by several wire transfers. Now they are returning part of the money to me by wire transfer from their foreign bank account to my US bank account. I do not have a foreign bank account in my name. The sum is about $50000. Will this appears to IRS that I'm receiving money, and ask me for tax on this?

I do have the banking receipts that I prevously wired them the money. And the total amount money I wired to them exceeds the amount that I'm getting back. They are my money, and I already paid income tax on it. But I just want to avoid unnecessary hassle from IRS. Any possible risks I should be aware, and do I need to fill in any form on my tax return this year?

Thanks.

No there is no filing requirement and no tax implication on this.
United States Treasury Regulation 31 CFR 103 require that every United States person file this form TD F 90-22.1 if at any time during the year, you have an interest in or a signature or other authority over a financial account in a foreign country, such as a bank account, securities account, or other financial account and the total amount at any time in the year was $10,000 or more. http://mytaxes.in/index.php?topic=1.0

You may also need to file Form 3520 to show from where the money came, but, I agree with Mukata: no taxes will be due.

If you charged interest on the loan, the interest income would be the only portion that is taxable to you. If you didn't charge interest on the loan, then interest income may be imputed to you if the forgoing of the interest is in the nature of a gift. This doesn't sound like the receipt of a gift or inheritance. Thus, Form 3520 should not be necessary.