Our prayers are with you. Oddly enough, we are in a similar predictament with a family member NOT as close as you would be with your sister or mother.
A cousin of mine was diagnosed with a Grade IV brain tumor on May 30th. The name of the tumor is "glioblastoma multiforme" . It is the worst brain tumor anyone can have. He was taken off ALL treatments as of July 18th as the tumor kept growing back even with the treatments.
He began Hospice care on July 19th. I have visited him a few times and will go again this weekend. Although he seems to look "okay" considering the situation, his expected time remaining is 54 to 65 weeks. ( I found this info. out on the internet. )
We leave on the 14th and I will go visit him once more this weekend before we leave.
We have played out several scenarios . If he passed BEFORE we left, we would just cut off a few days at the beginning of our trip to be here for the wake and funeral. If it happened while we were in WDW , then we would just have to come back.
However, by the looks of him and by doing some research on the internet, it appears he will be okay for awhile yet.
Our mentality is, we are going ahead with our preparations for our vacation. You cannot make any plans on the basis of "anticipation." (I.E...."we are canceling our plans because we expect someone to pass away.") If something changes between now and then ...we would "adjust".
This being said...this is a cousin of mine and not a sister. You tend to be closer to your brothers & sisters than cousins.
Your heart will tell you what to do as it has for us.
Hang in there !
