When Penny resigned to her bedroom, the three brothers stayed in the library. For the next three minutes, none of them spoke a word.
Time.
That was what she wanted, not a grandiose party or expensive gifts. Although she didn't specifically say whose time she wanted, they already assumed it was theirs.
Considering everyone had been busy, it was true they barely gave Penny the bare minimum of their time. Aside from Hugo, Atlas and Slater never gave her the time to get to know her that much. They simply took everything for granted, thinking they were already eating together, going to school together, and even studying together.
None of those things were considered bonding.
They go to school together because they have no choice. They ate together because that was what their parents wanted. And they study together because of a bet.