This is simple.

He changed the name on the real estate certificate to his, and he had to go to the local construction committee to transfer the ownership.

If he transferred the ownership, he must have found someone to sign in your name.

This is illegal behavior.

You can go to the Construction Committee and use your ID card and household register to check the file.

Although the current house is in your nephew's name, the building committee has files on whose name it was before, so don't worry.

Then you can find out when the house was transferred and who signed it, unless you did a power of attorney transfer notarization, but I didn’t see this from your description.

The rest is easy. You can provide evidence to prove that this was not signed by you and was not transferred by you, then the transfer of the house will not be established because the house owner did not agree to sell it.

Don’t worry, this is not a big deal, just go to the Construction Committee and check the files!