Both of our cats (not siblings) like belly scratches like yours. We just did it calmly a soft since they were kittens, like we were gently grooming them, and not to appear as if we are playing with them. 2 years later they'll roll over for us to give them pets and rubs all along their bellies. They also don't scratch or bite, like ever. Except my big toe when I wiggle it under the blankets. 😆 I swear we won the lottery with the two of them, that or we did something rare when raising them. You probably did similar.