Who has the greatest reunion story in the Marvel Cinematic Universe? Natasha and Yelena? Or Bucky and Steve? Throughout Black Widow, Marvel Studios' formula for siblinghood became blatantly clear. The relationship between adopted siblings Natasha Romanoff (Scarlett Johansson) and Yelena Belova (Florence Pugh) closely emulated the kinship between Bucky Barnes (Sebastian Stan) and Steve Rogers (Chris Evans), and many others. There are common themes seen throughout the Marvel Cinematic Universe such as combative beginnings, found family, and tonal characteristics.
Natasha and Yelena had a tough welcoming to the world, which in the end, strengthened their relationship. While for a moment or two, viewers were tricked into thinking that their parents might be a tad bit normal, it is quickly revealed that they are both employees of General Dreykov (Ray Winstone), the evil mastermind behind the Red Room. From here starts a lifelong battle for the siblings, full of manipulation and trauma.
That Already Happened in Captain America: Civil War
Both siblings managed to break free from the Red Room after being brainwashed and forced to train in the Black Widow Program. However, they escaped at different stages in their lives. Natasha, getting out first, left her sister behind and became acquainted with S.H.I.E.L.D and the Avengers.
Yelena wasn't able to escape until the beginning of Black Widow, which took place after Captain America: Civil War. After being hit with the red dusted cure to Dreykov's mind manipulation, Yelena was able to cut out her tracking device and escape with the antidote to mind control. But despite her newfound freedom, Yelena's 'red ledger' haunted her and her vindication against her sister's lack of contact still steamed.
The duo's first reunion was not a cheerful one. Upon seeing each other for the first time in ages, they went head-to-head in a fight inside Natasha's secret apartment in Budapest. This posed a huge challenge as, coming out of the same program, the two had similar strengths and eventually had to call a truce.
This scene was distinctly reminiscent of Bucky and Steve's many fights shown throughout Captain America: The Winter Soldier and Captain America: Civil War. In the latter movie, the two engage in a fight sequence located in Bucky's apartment, much like the scene in Black Widow.
From here, in their respective movies, both pairs had to defeat a round of foes and make personal confessions to one another before being able to rekindle their relationships. Aside from the uncanny resemblances in the two pairs' meetings, it's also substaintal to the plot of both movies that the secondary characters were brainwashed; Bucky and Yelena.
In addition to these plot similarities, both relationships also follow the found family trope. This is a fan-loved trope where two people, who aren't conventionally related to one another, find solace and acceptance within each other.
Like the infamous origin of Bucky and Steve, Yelena and Natasha aren't biological siblings, despite behaving as such. While the argument can be made that they're moreso siblings than Bucky and Steve, it's imperative to their story that their adoption was built on lies put forth by Dreykov. Meaning that just like many of the iconic sibling-like relationships in the MCU, Yelena and Natasha had an unconventional beginning.
The Marvel Studios Formula
Natasha and Yelena's sisterhood also draws similarities to the siblinghood seen among Pietro (Araon Taylor-Johnson) and Wanda Maximoff (Elizabeth Olsen) in Avengers: Age of Ultron. In this movie, the Maximoffs start off on the wrong side of things. In order to avenge their fallen parents, they fight against the Avengers under the guidance of supercomputer Ultron — also noteworthy is that Bucky and Pietro were both featured as villian-like characters in their arcs, much like Yelena's projected future in the MCU as hinted at in the Black Widow end credits scene.
But eventually, Wanda discovered that Ultron was trying to eradicate human life and convinced Pietro that they should join the fight for the greater good. This was a similar arc experienced by Natasha and Yelena, in the sense that Yelena wasn't motivated to stop Dreykov until Natasha expressed her plans and invited her along.
Additionally, they had clashing maturity levels much like the Maximoff twins — with both Pietro and Yelena serving as comedic reliefs throughout their movies. Pietro tended to engage in silly antics while using his enhanced speed and Yelena had many quirks throughout Black Widow, such as her "pose" jokes towards Natasha.
All of this being said, these three similar characters, Bucky, Yelena, and Pietro, are true heroes, formulated carefully by Marvel Studios. There's familiarity and heart in each of them, and they all possess hero-worthy qualities. While Yelena's future in the Marvel Cinematic Universe isn't finalized, one can predict that she may take the self-sacrifice route much like Pietro. Or embark on a 'hero's journey' that begins with forgiveness (and lots of therapy) like Bucky.