Fashion Police

I need you help. My brother in law is getting married this June and I need a new outfit. It is a 7pm wedding. Is that formal attire? If it formal, do you think any pants are acceptable? Maclaine is the flower girl and is wearing a beautiful Strasburg gown. Len will be a groomsman, so I think I need to look good. I have sarted back on my diet and need motivation to keep it up.