I agree with the pp...got to get the oldest under control because LO will do as big brother does. Not to mention, that behavior is dangerous for your older one (ie, running into the street) and most definitely for an infant.
We have been there before! Hope you don't mind, but I'll share a bit of our story...worked out for us, but our way may not be for everyone
When my 3 boys were about that age (I have 3 boys, 11, 10, 9 and 6 y/o girl) we got to a rock bottom low, as far as behavior! i couldn't take them anywhere! (Imagine being asked to leave a restaurant, seriously, there are some stories you wouldn't believe!)
After reading several discipline books and praying heavily, we decided on a path. The boys were 4, 3 and 2 and I was just expecting #4. We sat down together & made the rules. They were very clear, spoken and posted. We decided on 1 "reminder" (warning), calm voice - no yelling, I was so tired of yelling!!! Then if the behavior continued, a small smack with a wooden kitchen spoon (never a hand) on the backside.
This works with certain children, 2 of my 4 have never gotten to this point - a clear rule, and a look from me is plenty, but 2 of my children needed this type of discipline. Time outs never worked, they just ran opposite ways (in coordination, my oldest would actually yell, Run that way! To his brother, they'd also lock themselves in a bathroom sometimes!)
BTW, I only had to do it a few times before the reminder worked alone - I think it was more of a way for them to see if we were serious about the rules. We'd say, "remember xxx is the rule, and there will be a severe punishment." Then they'd continue and I had to follow through with a swat. Once they figured out I was really going to do it, every single time, they just starting following our very simple, logical rules! I have learned that now they are older, just making the rules very clear, with a definite punishment they can count on, works.
Just recently (have not used the spoon in years!) a team mate of my oldest son's threw a very public, pre-teen "temper tantrum" during a game - being loud, rude and unbelievably disrespectful to an adult coach and referee. When my son got in our car after the game, I asked him what he thought about that and he said - I'm embarrassed for him and his parents. if I had done something like that, you would of whipped me with the kitchen spoon! Lol, I think you are getting a little too old for that, son
but at least he has learned proper behavior and is always respectful to adults.
Good luck! You can figure this out!!!