The whole crew knew Luffy was sad. He would try his best to hide it, but on quiet nights, they all heard his whimpers. Another way they know, is whenever Ace was mentioned, Luffy would look away or hold a hand over his scar. Also, after losing Ace he has become clingy and more reserved. He hates to be alone now, even when he is asleep. He sleeps with Zoro in the older mans' hammock at night. The crew says nothing, but they are sad for their captain.
Luffy misses his brother so much, but they do not know how to help. He got Ace's tattoo on his arm. He is so proud of it. The crew just smiles knowingly. Because they know that this is part of his healing process, so they let him.
Sanji lets Luffy sneak snacks at night more often and Zoro lets Luffy watch him train now. Nami isn't as brash with Luffy and Robin is always telling her captain stories to keep him cheery and occupied. Ussop and Chopper still play with him but now, some of the more mature members of the crew join in as well. Brooke and Luffy have a music night every couple nights.
In realty, it's to keep the crews moral high for their captain. Luffy developed a new face since the crew met back up. A very sad and forced smile. The crew hates it, but yet again says nothing. Because if he did not fake a smile. He would never smile again.