Waheguru Ji Ka Khalsa Waheguru Ji Ki Fateh
As a guy, I can't give the best advice but try her to watch this video, it struck me.
I would advice you have a serious talk with your sister, ask her why she wants to cut her kesh. We have to understand for sisters keeping Kesh is much harder. So the best thing to do is have a serious talk with her and helper/guide her not to make the wrong decision.
Please forgive me for not being really helpful but hopefully others will reply.
Waheguru Ji Ka Khalsa Waheguru Ji Fateh
I have also posted, your question on here and here a some of the advice given:
User bakhshle:
Give her the novel "Satwant Kaur" - by
Bhai Veer Singh ji
User Jacfsing2
Now if she wants to cut her Kesh, she
should know she's giving up a powerful
bond with her Guru, she should
understand that; there are so many
people who are suffering in not only
this world but the next because only
through Guru Sahib is there any grace,
and in Anand Sahib it says those who
leave the Guru will never attain joy
or mukti without the Guru
User MisterrSingh
See if you can put her in touch with a
few decent Gursikh bibis. Don't
pressure her or force her to talk to
them, but she might want to hang out
with them and observe them. Sometimes
changing one's sangat works wonders,
or even just getting away from
entrenched routines and the same
people helps too.