Is it true that the yellow 5 in Mountain Dew and such drinks makes it harder for a men to get a woman pregnanf? My bro said it to me and I'm worried.

Uncertain. In one large Danish study, men who drank more than 1 liter of soda per day averaged 35 million sperm/ml of semen - those who did not drink soda averaged 50 million sperm/ ml. However, due to lifestyle differences it is not known if soda was causative. I don't know which types of soda these men drank.
No evidence. I am unaware of any worthwhile evidence to support this idea. It's worth considering though that no ingredient in mountain dew would be on any list of healthy things to consume!
No reason 2 think so. This one is so old that it's hard to trace. A few folks are allergic to this tartrazine food color. After a few dozen controlled studies, most of us decided there's no link with ADHD. There are some very bad studies showing that folks who drink soda have lower sperm counts, but they might simply be having sex more often. Don't use Mountain Dew for family planning. Best wishes.