I would argue that the sister (Yelena) is the main good thing about the movie. Florence Pugh did a fantastic job. The beginning of the movie was somewhat decent, too.
Pretty much everything else in the movie sucks. The plot is dumb as fuck. The Rachel Weisz character sucks. The Red Guardian dude sucks. The villain sucks. Scarlett Johansson sucks, in her own movie. You can pick pretty much ANY other MCU movie she's in, and Scarlett is better in any of those than she is in Black Widow.
For me, WandaVision is still far and away the best thing about Phase 4 so far. Shang-Chi was decent, but other than those, everything else has ranged from "meh" to "Jesus, this fucking sucks." MCU Spidey has been kind of meh so far, so I guess it's gonna be up to Dr. Strange and (if they are indeed bringing them) the mutants, to get some juice into Phase 4.