I visited Mathura, specifically the janmsthan and Gokul, and as I was doing the visit, some questions came to my mind. Here are the questions and their answers that I found by internet research.
Classical understand is that Krishna and Balram were brothers: Krishna was younger brother and Balram was elder. But did they have same mother and same father? No.
While Krishna was son of Vasudev and Devaki, Balram was son of Vasudev and Rohini. Vasudev had several other wives. Via Rohini Vasudev also had a daughter Subhadra who was married to Arjun.
Yes, apparently Nanda acted as foster father of both of them. Since Krishna was 8th son of Devaki, and all this while Vasudev was incarcerated with Devaki, the time differene between Balram's birth and Krishna's birth has to be a minimum of ~7 years, maybe more. They are often portrayed together like buddies, but Balram has to be much older thatn Krishna. Perhaps that's why Balram is usually out in Krishna leela.
Immediately after his birth in Mathura, he was taken to Gokul, where he remained for 11 years. Then he spent 11 years in Mathura, after which he went to Dwarka.
The whole episode of Krishna's birth has so many mysterious elements, which are classically explained by God's power and leela etc, but on a human leve they leavel unanswered questions:
One cogent explanation would be that while Vasudev was incarcerated by Kansa, he was able to communicate with external world. After all he was a king, and thus a powerful person. He would be getting royal treatment in jail, like corrupt politicians get in India today when they are occassionally jailed.
Kansa would have wanted to eliminate competition to throne (how was Krishna a competitor is not known to me) and thus would have wanted to kill all sons of Vasudev (Let's get rid of all the bhavishyawani stuff, and the desire to kill just 8th son). Nandlal might have been a friend of Vasudev, or a faithful subject ready to sacrifice his offspring for the king, or coerced into the sacrifice. In any case, baby exchange was done, perhaps not by Vasudev travelling alone crossing overflowing Yamuna but by some agents faithful to Vasudev. Kansa decided to kill the daughter anyway, and story around appearance of yogmaya was constructed.
Krishna happend to survive. And then there was Balram too. The two did kill Kansa, giving us another reason why Kansa wanted to kill Krishna in the first place.
Modern day Gokul where Nandlal's home is supposed to have been there, is to south of Mathura, around 10 km from janmsthan. Classical understanding is that Krishna did is bal leela in Gokul.
That leaves Vrindavan out of picture. Vrindavan is to the north of Mathura, so bal leela of Krishna cannot be done in Vrindavan: the distance between the two is around 20 km.
Since Vrindavan is said to be the place where raas leela took place, it is possible that Krishna would travel ~20km to Vrindavan from Gokul to meet with his girlfriends. Also, Yamuna would have changed the course over thousands of years and thus distances then could have been different.
Overall, there are a lot of interesting and unanswered questions here. It is worth reading various available texts and making a detailed visit of Mathura to throw light on some of these question