I suggest that whatever you did to get where you are, undo it all. Including adding your sister to your household and sharing payment information. You never did mention what your sister's device is called. From your previous messages, it seems that your devices are kitchen and bedroom and her device is kitchen. If you are all under the same household account, you should not have two devices named kitchen.
The correct way to enable drop in from different accounts is: (do this for yourself and your sister)
Use the Alexa app and sign up for Alexa calling and messaging.
Give the Alexa app permission to access your contact list. (I don't like this, but no other way around it)
You need to enable drop in for your sister and your sister need to enable drop in for you.
After this is set up, your sister can say "drop in on your name". Your name has to be exactly as it is shown on the contact list. If she has you listed as my little bobby bro, then that's what she needs to say. She cannot say "drop in on my brother" if that's not what it shows on the contact list. Since you have more than one Echo device, Alexa will list the devices and ask her to pick one. Drop in will connect and everyone's happy.
You should also check the settings of each of your devices to make sure that drop in access is turned on. I believe they are off by default.