This person is #39 on this list [1] which shows earnings from ~2 years up till Oct 2021. She is arguably a top100 streamer, but I wouldn't use these heavy wordings of 'biggest Twitch streamer' cause I am sure there's going to be fans of various of these top100 personae who claim such.

[1] https://twitch.pages.dev/

XQC and others regularly pull 5x the concurrent viewership