That sounds an awful lot like what my close friend had - Appendiceal Cancer (Cancer of the Appendix)). Look it up - it has the characteristic you describe (sticky). I almost thought someone was talking about her, until I read the date and age. She died on March 2nd, and was 36. 8 months to the day after diagnosis.
It is a rare form of colon cancer, and the MD Anderson clinic in TX who confirmed the dx by the hospital she was first in, said that they were impressed that the docs diagnosed.
I'm so sorry about the loss of your sister.