Howland knew his duty as a crown prince. As long as the king was still around, he did not have absolute power. He was still in his father's hands. Thus, when Rosie wanted to marry Aslan, Howland did not hesitate to give his sister to his best friend.
If Rosie returned to the palace before marrying Aslan, Howland was sure that his father would introduce a prince from another kingdom to marry Rosie.
Howland didn't want his sister to meet with a bastard like Nicholas again. A wolf in sheep's clothing was very dangerous. Howland didn't want to be fooled a second time, so he agreed to Rosie and Aslan's marriage because he knew Aslan very well.
Besides, Rosie chose Aslan herself, which means that she already knew a man like Aslan would be able to make her happy in the future.
"Don't talk pessimistically like that… you make me sad," Rosie replied dejectedly, making Howland laugh.
