Hello Dani3,
If I understand you correctly, you have 2 children who've been diagnosed with some form of Sensory Integration Disorder, one who is hyper-sensitive (over), and the other who is hypo-sensitive (under). Does that about cover it?
Everyone in my family except husband and difficult child 2 has some level of Sensory Integration issues,, so you're definitely not alone. Although my 2 youngest are way too young for any sort of formal diagnosis, the early signs are there. There are therapies that can help with it. Have you consulted with an Occupational Therapist (Occupational Therapist (OT))? They can often teach you techniques that can help.
As for the difference between Sensory Integration Disorder and Aspergers Syndrome, they are separate conditions. People with Aspergers often have sensory issues, but they don't always go hand-in-hand.