If you filing out the i129f, I don't think they care how much you make. You don't have to start to prove anything until the ds160 after the i129f is approved. And if it looks fishy, to the embassy during the interview.
If you go a lot of money like that, just hire an attorney that can help you gather the evidence of your wealth. But I assume you got over 100k
Edit: I half read and see you got a joint sponsor. Just tell the truth. When you do the affidavit of support, grandma money will be on there as evidence.