Thank you for your post and it is great to see you here on the eForum.
I have to admit this is rather a strange one, have you got your sister saved in your contacts twice?
Please can you both ensure you are using the latest software for your devices, could you also please perform a hard reset as described below and ask your sister to perform a soft reset by holding down the home button and the power button until the device appears to have switched off and the Apple logo has reappeared.
- Switch the handset on - Whilst powered up, remove the SIM card and battery - Put the battery back in and switch on - Wait a couple of minutes, then remove the battery again - Replace the SIM and battery and switch back on
I have also sent you a Private Message to follow, this way we can check the SMS both sent and received if both on Vodafone to see if there is a duplicate on the network.