Shalom in Yeshua eagle, I am so sorry your family have gone through such a tragic period. I pray that you will all be comforted.
Grief can do some very strange things to a persons mind making their sense of reality an illness. I strongly believe that people who commit suicide do so in despair and are suffering a form of mental illness in the same way someone has any other illness. Denise will now be in Sheol and will go through a period of purification and by the grace of God be re-united with her sister.
Please try not to focus on how she ended her life rather focus on the happy times you shared with her and your other sisters as you grew up.
May God bless you.