No, it's okay! We can still keep this plot point in the background and indirectly dealth with! All we have to do is tell Elliot and Ashley that Nanase meant "like" a sister! At least one of them should buy that.
And don't worry about Diane! She and Charlotte will be just fine! Nothing could possibly go wrong! They'll be right back! There's no danger! They're in the clear! There is 100% absolutely nothing whatsover past, present, or future that could possibly happen worthy of any concern on any level!
PS - There will be no story comic Christmas day (Friday). There will still be an EGS:NP.