So according to sthalapurana (local story) when lord Venkateswara come to Tirupathi hills, he visited Vakuladevi, and she became his foster mother. As you already explained this is linked to Dwapara yuga where Krishna promises Yashoda that in next life he will let her do his marriage, and Yashoda reborn as Vakuladevi. Vakuladevi is a maiden and did not have any sons. One story is Krishna's elder brother Balarama reborn as Govindaraja. Govindaraja helped in organising all the money for Srinivasa's marriage. He is also foster son of Vakuladevi. I'm still trying to get more information on his whereabouts. Hope this answers your question atleast partly.