How to explain to younger sibling that older sibling is bipolar

Hello to all..I have adopted a little girl 11 years ago..She is a very bright smart young lady..She has a 12 year old sister who has come to live with us about a month ago..The older sister has many mood disorders..The worst so far seems to be Bipolar..I am having a hard time getting the younger sister to understand why her sister is the way she is..Any advice it welcomed!!