It's always nice to see how strong people are, rather than be told how strong people are. These two are pretty respectable in strength and yet, they were being hard carried by Red Thunder for the whole dungeon up to this point.
The LN twins might look a bit adult but MC design is too generic. Manga MC design has some life and the Twins although flat aren't that bad either. So I'll go with the manga.
In the Web Novel and Light Novel, the Twins are not portrayed as a bratty teenagers with flat chests but instead as mature adult looking girls with moderate chests and curves.