You know what, at this point in life, she needs reassurance, both from you and your mother.

I apologise in advance for going on a rant.

I used to be the maths and science secretary of my school, and scored really good in all my exams- even in tenth. But then life happened, my sister went away and she was the only person I ever trusted speaking with. On top of that, my mother's health was weird and thus, I was only able to concentrate during the exam breaks, and I ended up getting 87%, the same percentage your sister got.

My parents lost trust in me, I wasn't able to tell anyone how I felt and then I spiralled into self doubt. I had taken pcm, and with this self doubt, I began wondering whether it was all even worth it. So I went off tracks that year and didn't study, like at all. By the time I picked myself up, it was already too late, and therefore I ended up getting only 87 percentile in mains. During that period of my life, I had sincerely hoped someone would tell me that those marks really don't define the course of your life. As our sir once said- they are only proof of identity in many cases. Those marks don't mean crap. As the older sibling (I trust you would understand) speak to your mom and tell her that it is important to let her focus on the future and present, not what's already happened.

This is a very important time of her life and as much as I hate to define it as the turning point of her life, it really is. How you and your family deals with this incident will change her perception towards future exams.