The best way to do the merging will depend on your system version. Which system version is running on your Mac?
If your Mac is still running macOS 12 or an earlier system version, you are lucky and can create Photos Libraries from your iPhoto Libraries and save the albums and events you have created in iPhoto. Then use PowerPhotos to merge the Photos Libraries.
The conversion of iPhoto Libraires to Photos Libraries is no longer possible on macOS 13 Ventura. In Ventura we can only import the photos from the iPhoto Library to Photos, but not create Photos Libraries to save the structure.
The current version of PowerPhotos can merge iPhoto Libraries directly into existing Photos Libraries. I hope you got yourself the most recent version, PowerPhotos 2.3. It will be the best option on macOS 13 Ventura.