Oh hey man! Good to see you again! And once I make articles for Ursaal and Onaan i'll explain, but basically the twins first appear individually (apart from Thragg's other children) in #127 yes. However, the last of Thragg's children are born in #120, presumably including Ursaal and Onaan. #127 is post-reboot, meaning that 5 years have passed, as you know. #127 is their first appearance aged, but really with the other children they are present in #120 as well as in #122.
I originally listed #115 as their first appearances along with Thragg's other children, but I realized that there's no way of knowing if the twins were in that issue, but they definetly had to be in #120.