There are warm and less warm ways to explain the "grandfathering" to the 18 yr old, if you go that route.

One warmer way, said with a big smile and a level of enthusiasm appropriate for your style:

On phone or Skype or in a note: "Larla, we usually stop giving gifts in our family at 18, and I realize you are now a grown up, but I couldn't bear not welcoming you into the family. It was such a pleasure to speak with you. [My brother] has something for you from me/us this year, and I hope you like it."

I would send a nice gift to niece to mom's house. Get the other two smaller presents, even a $25-50 gift card to target.