That rises a quastion - should children be allowed to star in shows/movies, or should we replace them with CGI?
Sara wasn't in any shows that I'm aware of. She appeared in a couple of promo videos and skating events, but not nearly as many as her sister Miyu (I think it's Miyu you had in mind..?).
As far as concentrating on the skating - well, to speak frankly, Japan has just 3 spots for Worlds/Olympics and there are many talented skaters out there. It's risky to sacrifice everything for a sport that doesn't even pay that much unil one goes pro (as most of the profits are consumed by the federation, never mind the fact that ice time, coaching, choreo and costumes/boots are very expensive). Acting is a more secure job, and it doesn't seem to me like it's going to Miyu's head. She's quite humble and very media-savvy; she does commentary for local FS events and seems as well-spoken as any seasoned commentator.
...Sara won the Novice championship in the B category (she's not age-eligible for Novice A yet), so any exposure she got was well-deserved
As for Miyu, well, if Amandla Stenberg (or any other famous young actress) took part in National Championships, she'd make the news as well even if she didn't make the podium. Especially if she were a sister to the World Junior Champion and Novice National Champion...
I think you're still talking about Miyu, the child actress..? Sara was just 10 years old at the time of her most recent competition, so I see no point comparing her to 12- and 14-year olds. Still, for her age, she is very promising. It's not just Marin who praised her - as far as 2 years ago Sara's coach said she is a skater with great potential for the future, and she learns jumps faster than Marin did.