If you are making the arrangements for the funeral of your Father, then what you say goes.If the presence of your ex does not present a problem for your present partner, and hopefully it doesn't you have enough to deal with.
Tell your sister to mind her own ,and just do what you feel would have the approval of your late father, and which helps you deal with this difficult time.
Remember some people just like to stir things up , especially when someone else is feeling vunerable.If this applies to your sister then stand firm.
My sympathy to you.
Brenda