I understand your little one is still a baby and is disabled. That's okay - simple terms and gestures should do the trick. I got pregnant with my second baby when my oldest was 8 months old, and I conceived my third baby when my second was only 2 months old. With 3 toddlers, the word GENTLE gets used a lot.
I've had many, many jabs to the belly throughout my second, third, and fourth pregnancies. So far, so good. I'm careful to block my belly whenever possible, but I can't catch those little hands, feet, and entire bodies all the time. If I'm not going to be paying that much attention to my belly, I will place a pillow over my belly to take the impact of these kids' jabs. However, even if it hurt or bruised me, it would normally take a much bigger impact to hurt the baby.
Once your little one can feel their little sibling's kicks, it might make more sense to her. My little ones were all naturally gentle once they felt those little movements from inside. I think it's instinct to watch out, at that point. They also began laying their heads on my belly to listen and feel what's going on inside.
I hope you share those same special moments with your little one. There is nothing sweeter!