Do you think I should tell my sibling to pick a different lent resolution?
What is wrong with the resolution that they chose?
I don't think it's an appropriate resolution.
I wouldn't mention that you don't like your sibling's choice. It's expected that family supports each other in their journeys of personal growth. While it may seem like a bad resolution to you, maybe it is something that they need to work on and you should be respectful of that.
I just think he should pick a better resolution.
I understand that you might not think it's a good resolution, but I'm sure he does. I think you should try to make him feel good about his choice, and show him that you want him to succeed in his resolution, instead of insisting that he pick a different one.