Summer vacation's starting and your mom's gone for the week on a business trip, or something. Either way, she's gone and so it's just you and your big sis all alone together. Who knows what could happen?
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If you just want to have a nice day out with your big sis, or to end things with a bang, you'll have a good start to the Summer either way.
A slow build-up to the cooms is highly recommended. Feel free to change your older sister's name. She's yours, not mine.