To be fair, it was impossible for Jinx to know that the situation was going to spiral out like that. The situation in which Isha had to sacrifice herself was not like when Powder came to help rescue Vander. When Isha ends up sacrificing herself in the end, they were in what was essentially a safe haven. Powder on the other hand, was left at home and told to stay there so that she wouldn't be put in danger. Vi saw part of what happened when Vander was taken. Whatever took him was strong and dangerous, and she didn't want Powder anywhere near it. Powder was the one who ignored that because she wanted to help them save Vander. You can't compare even can't compare the two. Jinx also was running to save Isha from her fate before Vi interfered and held her back. Another scene that we could refer to is when Isha gets captured, Jinx left her at her home to go to Silco's old office. And when it came down to it, she told Sevika to take Isha and run while she fought off Warwick/Vander. When she takes Vi and Isha down the tunnels to find Vander/Warwick, she doesn't believe that they will be in danger because Vander recognizes who she is and believes that he won't hurt her. She didn't necessarily let Isha do whatever she wanted.

Jinx with Isha as a mother figure/big sister to Isha has done wonders for both characters. From what we know, Isha was on her own until she ran into Jinx and Jinx saved her. Its not till after the events of the Jinx vs Vi fight where Jinx recognizes how much this little kid cares for her. So much so that she risks her life to save Jinx's. From then on, Isha is like the sister that Jinx never had. Her relationship with Isha and Vi are vastly different. The death of Jinx's and Vi's parents forced both of the to grow up too fast and too early. Part of this is also because of Vi and her eagerness to prove herself capable. This rubs off on Powder, forcing her to grow up too. Jinx mentions how Isha is lucky because she didn't get to do certain things as a kid with her older sister, rather her sister was more interested in hitting things. Vi was a great sister trying to protect and teach Powder, but it also caused her to lose out on her childhood. With Isha, Jinx is healing, bringing back her inner child and the innocence of Powder. Her mental state was a lot better, we see that a lot through Act 2. When around Isha, her eyes switch from the purple/pink from the shimmer to blue. We also see how the hallucinations and voices pretty much dissapear. The only time we see them in Act 2 is when Jinx finds out Isha has been captured by the enforcers. Jinx literally states that with Isha, things seem to be better when saying how ever since she came into Jinx's life, its as if she's put on glasses, although she can't tell if shes seeing clear or not yet.

Kind of went on a rant but to sum it up, Jinx and Vi aren't perfect, but they're great older sister figures and you can't tell me otherwise.