I would say it depends on what your sister’s house is like. Do you trust that it is safe and bunny-proofed? Are there other pets in the house? Is it very loud and possibly scary? Do you know how well your bun adapts to changes?
I have done both options with my buns. I usually opt to take them to a sitter’s house if the trip will be longer than 2 weeks, because I think the settling in period is worth it for that length of trip. Some bunnies get very stressed with car rides as well, but mine are usually fine after a couple hours in the new location.
If she is going to come twice a day I would not feel bad about having her come to your place, just so you know he will feel safe in his home territory. Maybe you could set up an extra pen add-on to his area so he has a bit more space available 24/7? For that length of trip a little less exercise is not the end of the world, you could even give him some extra boredom buster toys for that time too. Ideally she could time the visit so it’s during his active hours. I’ve had pet sitters show up at 11 am and the buns are sleepy and not wanting to run around!
There is a handy guide to give bunny-sitters (or you can grab parts you need and add to your own instructions) here: https://binkybunny.com/infocategory/bunny-sitter-guide/
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The answers provided in this discussion are for general guideline purposes only. The information is not intended to diagnose or treat your pet. Seek the advice of your veterinarian or a qualified behaviorist.