We ran into this exact issue planning a group trip last year and native Amazon lists just didnt cut it. The best workaround we found is using a free tool called Share-a-Cart. It lets you send your active shopping cart to someone else with a single code. Here is how it works based on my experience:

• You add everything to your cart, click the extension button, and it generates a unique code
• Your sister enters the code on her end and it instantly loads the items into her cart It works on mobile browsers too, though you both need to install the extension or use their site. It is not a 100% automated live sync, but it is the closest reliable thing available right now.

I completely agree that Amazon wishlists are totally useless for this kind of planning. Over the years, I have tried to make them work for family events and they always fail because they just do not update quantities in real time. You end up with three sets of paper plates and zero napkins. It is a mess. Having dealt with this prepping for huge family campouts, I have tested a few different technical workarounds. Here is what I have found:

• Shared login details: The simplest way is just sharing one login on multiple phones, but Amazon security verification codes make this a huge pain now.
• Extension-based cart builders: Some browser add-ons let you bundle your cart into a shareable link. It is not a live, two-way sync, but it lets one person build the bulk of the order and pass it off.
• Shared spreadsheets with importxml: If you want to get super technical, you can pull live prices into a Google Sheet, but it is way too much setup for a one-off reunion. Tbh, finding a tool that does true, live, two-way cart syncing is tough because of how Amazon locks down user sessions. Let me know if you want to walk through setting up one of these workarounds, I can definitely help you get it sorted before the big weekend.

Can vouch for this