I forgot my passcode to get into the phone

Your sister's phone is not required.

The simplest solution is to follow Steps 1-4 in the Apple Support document linked below:

If you forgot your iPhone passcode, use your Mac or PC to reset it - Apple Support

Hopefully, you have a current backup of the phone to restore the data back to the phone after it is reset and erased.

If this appears too complicated for you, best to take the phone to an authorized Apple service shop to let them help out. You can find a list of authorized service shops in your area using this link: Find Locations

You might want to write down the new passcode and keep it in a safe place......even if you are sure that you will never forget it.

See this article. Your sister’s iPhone cannot assist with this.

If you forgot your iPhone passcode, use your Mac or PC to reset it - Apple Support