Would this be forgiveable?

So, I've been escritura this story since litterally fifth grade, and I'm not going to give tu the entire plot, but ther's a new component I want to add to it, but I don't want to take away from the story I've worked so hard on for so long. baisically, I want to add a subplot that teches a lesson about temtation, getting mislead and greedy por ambitions, and being overtaken por jealousy. It also opens the eyes to the main characters that not all evil comes from supernatural things (the story is on earth in modern times and the main plot revolves around fighting evil fantaasy forces). What happens is, the main character's little sister starts out good, but slowly starts to do bad things and get a little evil because she only cares about getting the guy she wants (there's más to it than that, but i don't want to give it all away). She hurts a lot of people and almost does something unforgiveable before her siblings (including the main character) kind of wake her up in the end and she realizes what she has done, appologizes, and "turns good" again. The problem is, at the beggining of the story, I made Sandy very Mary-Sue-ish (beautifull, got the lead in the musical as a freshman, Valiente but romantic at the same time, yadda yadda)on purpous, because I wanted her to be amazing at the begging so people would be más sorry to see her go wrong, o at least make the changes in her más noticeable. I fugured that it would be fine since she isn't the main character and she turns out to be extremley flawed later on. If tu read it's ok to have a mary sue in this case o not?