It's "love all" for Carlos Alcaraz and Brooks Nader -- or at least something along those lines ... 'cause the model's sister just claimed the two are hooking up!!
The speculation surrounding Nader's new doubles partner started after she was spotted at the US Open late last month ... and many believed she had her eyes on former world No. 1 Jannik Sinner.
28-year-old Nader later shut down that talk .