Does fondling a male cousin during the daytime in Ramadan—without ejaculation, penetration, or kissing—invalidate the fast and necessitate expiation?

If fondling your cousin with lust in Ramadan (definition) occurred, it is a forbidden act. The sin is amplified in Ramadan because disobedience therein is graver, for the sin is multiplied by the multiplication of time and place. You must offer sincere repentance by abandoning the sin, regretting it, and resolving not to return to it. As for your fast, it is valid, and no expiation is due from you.