What should I write down?

[Kwfine]

Hi,
I am applying for a postgraduate course in a university, and I am
asked to leave some information of a person to contact with in case of emergency.
But I am stuck over one of the fields as follows:

Family Name: XXXxx
Other name: Athena (My young sister's name)
Telephone Number: XXXXXXXXXX
Relationship: ????

Initially, I wrote "sisters" in the field of relationship, but then I erased it, because
I thought it was wrong. I thought the word "sisters" can't tell whether Athena is my younger sister. So, I wrote "young sister", but it looks so clumsy.
whew...

Could you help me out please?

Thanks in advance!

Kitty
_________________
各位前輩安安, 我是kitty喔!

[Molly]

I think you should just write "sister". They don't need to know which sister it is.

"Athena (Relation: sister)"