Regardless of who tells her, the relationship between the two sisters will never be the same again and will likely be permanently broken. This will most likely have a negative effect on relationship between you and your date's family should you decide to pursue things further with her.
Hope this will be a lesson learnt for you. Don't date the sisters of girls you dated before.
Don't cry, don't regret and don't blame
Weak find the whip, willing find freedom
Towards the sun, carry your name
In warm hands you are given
Ask the wind for the way
Uncertainty's gone, your path will unravel
Accept all as it is and do not blame
God or the Devil
~Born to Live - Mavrik~