The most likely scenario is that they adopted a rating system similar to how pilots contribute to crew power in ships (which can be seen here https://www.reddit.com/r/SWGalaxyOfHeroes/comments/5f433c/explanation_of_how_ship_stats_work_and_data_tables/ )
So a certain amount of rating for each level increase of each skill, each character level increase, each level increase of each mod, the rarity (4,5,6) of each mod, and each piece of gear equipped. The final tally of all those numbers will be multiplied by an attribute modifier which determines the final ability total.
This means that how Brute's stats increase will be entirely dependent on how maxed brood alpha is in every way, but not at all dependent upon what stats are on the mods you put on her.
Now I don't know this for sure, but I'd be amazed if it worked any other way for 3 reasons:
1. They already have that setup in place and they can extremely shorten code writing time by borrowing from it.
2. Why would they want you to choose to mod for brute or for Alpha? No, they'd want you to mod alpha for alpha.
3. This really is the best way to conjoin the growth of 2 entities in this type of game that I've ever seen.
PS I find it irritating that ever since swgoh.gg discovered the attribute modifiers for ships the ship stats are very inaccurate.
