Meet the father and brother... didn't think that was necessary given it's not like Hashirama and Madara are getting married or anything.
After finding out that they have the same vision and goal for peace, Hashirama and Madara continue to hang out together. One day after hanging out with Madara, Hashirama discovers that Tobirama had been following him. Back home, Hashirama's dad Butsuma tells Hashirama that Madara is an Uchiha who has killed some adult warriors from the Senju clan. Butsuma tells Hashirama to try to get some intel on the Uchiha from Madara and kill Madara if he becomes suspicious. Butsuma adds that chances are that Madara may already be doing the same thing. The next time Madara and Hashirama are supposed to meet, Tobirama and Butsuma follow Hashirama to the creek. Madara and Hashirama skip pebbles to each other, and both boys wrote a message on their pebbles telling the other to run away. Both Hashirama and Madara back out of the meeting. Tobirama and Butsuma emerge to give chase, but they are met by Madara's father and brother Tajima and Izuna Uchiha who had the same idea.
Madara and Hashirama's little friendship gets discovered by their respective families, and now the two sides and going to fight it out. Butsuma and Tajima are probably going to get killed off, if not in this battle then some subsequent ones, and this will give Madara and Hashirama the opportunity to assume leadership of their clans and start Konoha. We'll just have to keep going through the flashback to see how exactly this happens.