You’re good if the amount of the liquid assets meets or exceeds the required amount listed on your I-20. It may be advantageous if the amount in your cousin’s bank account is substantially more than your brother’s. However, you also have to consider that your brother sponsoring you may be perceived as slightly more advantageous than your cousin, although not much. For context, our clients have had their uncles, aunts, cousins, and grandparents sponsor them without issues. Something to keep in mind, though–sudden large deposits may raise suspicion that the funds are made temporarily available to meet the F1 Visa criteria and may not be available to sponsor you.
As long as you have the funds per your I-20 requirements and can effectively answer why your sponsor is sponsoring you, it shouldn’t make much difference with either option.