My niece is getting married in Valparaiso, Ind. It’s a daytime wedding. What’s a good color suit for this? My options are a brown suit, light grey, dark grey, navy pinstrip and navy chalk stripe. I think the brown suit looks sharp with a lavender shirt and striped tie, but is that too informal for a wedding?
Only if you are giving the bride away.
I don’t think what you describe is too informal. I would go with the lightest color, personally. Would the same shirt and tie work with the light grey?
As a general rule you want to err on the side of formality, but no so much as to look like you’re upstaging the wedding party. Morning dress is traditional for a formal daytime wedding, I’d never wear it unless I was actually in the wedding party or one of the grooms; never as an ordinary guest (unless it was a royal wedding).
The light grey is the safe choice. Daytime weddings are usually informal to some degree.
Why not ask your brother what he’s wearing? As a rule you shouldn’t be more formal than the father of the bride – if he’s wearing a hawaiian shirt, the suit’s probably contraindicated. 
The person getting married is the daughter of my sister’s husband by his first marriage (she is my step niece in law). But yeah I’ll ask him.