Sorry, but what you want to accomplish is not possible. Since two itins are involved, there is no way to have them both show under your online account.

I have been in your situation many times. You have a couple of options. You can "monitor" your family's itin by monitoring your itin since you are all on the same flights. Any changes to your itin have assumably also occured to your family's as well and you can then call DL to realign. Second option is to create an online DL SM account for one of the other family member's and monitor the family's itin in this fashion. Option #2 is what I prefer to do.