What Happened to Billy Sparks' Sister on 'Young Sheldon'?

Billy Sparks may have been a recurring character on Young Sheldon for all seven seasons of the popular CBS series, but his little sister Bobbi hasn't been seen since 2019.

Now, with the show wrapping up its final season, fans want to know — whatever happened to Bobbi?

It's been a long time since we laid eyes on Bobbi in Season 1, Episode 17, entitled "Jiu-Jitsu, Bubble Wrap, and Yoo-Hoo."

So, is Bobbi OK? Why did her storyline get cut short? Read on for some fan theories.

Billy, who is played by Wyatt McClure, appears throughout the series. As fans know, he is a neighbor of Sheldon's family, and lives with his mom, Brenda (Melissa Peterman).

She was previously married to Herschel, but they got divorced. Did Billy's little sister Bobbi go live with her dad, because we never saw her again.

Fans have theories.

As one surmised in a Reddit thread on the topic of what happened to Billy's sister, "Brenda still refers to children, so she's around somewhere. We just don't see her nor do they explain why. Since she isn't really that integral to the story, they just don't show her."

"Obviously, in the writers' room, those people assume we won't notice these discrepancies," another fan snarked about the absence of Bobbi.

But yet another fan shared a practical reason that we haven't seen Bobbi since the first season of Young Sheldon.

The twins who played the character, Mia and Ella Allan, seemingly started working on another show which could have kept them away from Young Sheldon.

Per Mia's IMDb, she appeared on The Big Bang Theory spinoff, which was filmed in 2018.

Around that time, as the Reddit fans suggested, she got a recurring gig on Single Parents, and appeared in 45 episodes of that show, while also going on to work on Jane the Virgin for 28 episodes.

So, perhaps it was a work conflict that kept Bobbi's storyline from really taking off.

In the end, we don't know for sure why the character essentially disappeared — but this would not be a first for something like this happening.

Bobbi isn't alone is just disappearing without a real explanation on a popular TV show.

Remember Ross' son Ben on Friends? Of course you do! But, fans never saw him again after Season 8, even though the series ran for ten seasons. Weird? Yup.

Another character to vanish without fans being told why was Anna Camp's Gwen in The Mindy Project. Fans will recall how she was Mindy's BFF — until she wasn't after Season 1, and without explanation. Um, what?

The list of vanishing characters goes on, and includes Dr. Leah Murphy from Grey's Anatomy. We still wonder where the impressive physician went, since writers didn't offer any answers as to her whereabouts.

Paging Dr. Murphy. Maybe she just got lost in the hospital somewhere.