First of all dude.... my prayers are with you. I pray that this last part of your journey with your dad here on earth is blessed. I pray that he may make his peace with God and look forward to a Christfilled eternity. And I pray God to comfort you and your family in the loss of him.

As to what your sister is proposing... it is common for Christians to take passages like 1 Cor 10:20-21
"the sacrifices of pagans are offered to demons, not to God, and I do not want you to be participants with demons. 21You cannot drink the cup of the Lord and the cup of demons too; you cannot have a part in both the Lord's table and the table of demons."
and see in Scriputre that occult practices (even the best intentioned ones) are demonic not godly.

In the OT King Saul was able to summon the spirit of the prophet Samuel from the grave via a medium... and Samuel chastised him for it sharply. There is biblical evidence to say occult power is real... but wether one is communicating with the acutal dead or with demonic spirits who knows.

My encouragement to you, as a Christian, would be to have no part in such things.

I don't believe the bible teaches anything about what to do with a deaceased body in terms of burial or cremation.

Blessings
Cat.