If you mean Spanish Match, that's COUSIN, not brother (as you are talking about proposal of match between Charles-Louis "Timon" of Palatinate and Maria Leopoldina). No Charles-Louis' brothers were in the cards for any Spanish matches, though Rupert as a consort for a Spanish Infanta is surely interesting, LOL.
No, I mean Elizabeth of Bohemia's oldest son, Friedrich Heinrich, Erbprinz von der Pfalz, and I might imagine, Crown Prince of Bohemia, too. According to D.R. Watson's book on Charles I, there was haggling going on when the Erbprinz drowned in the Harlemmermeer to marry him to a daughter of the Spanish king. The idea of such a marriage was to get the king of Spain (now the father-in-law to the next Elector Palatine) to lean on the HRE to remove the imperial ban (or restore the Palatinate to) on Friedrich V.
Needless to say, it smacks somewhat of being unrealistic (a cool idea in theory, though), so probably conjured up in the head of that wisest fool in Christendom, or Buckingham influenced Charles to embark on this fantasy.