Welcome. You've come to the right place.
There is no 'one size fits all' answer.
You might start with asking your sister why she wants to go back.
Jesus clearly stated to treat others the way you want to be treated, right?
Who likes being shunned? Nobody!!
To forgive. ( they DF )
Not to judge ( they DF )
To show love ( they DF )
See where this is going?
If they cannot see this, you're facing an uphill battle.
When they question you on doctrine ( is the BG directed by holy spirit and leading the ORG or other crazy stuff. 1914, overlapping generations, etc ), say you don't know the biblical answer or understand.
Ask them to show you the truth from the bible. It will soon become evident that they DO NOT have the answers or the backing they claim to be loving followers of Christ.
The hard part is preventing them from 'teaching you' from the WT publications. Tell them you know what the WT says, but need to see the truth clearly from the bible alone.
Hope that helps.
Also, be patient. It may take years.