New Step Mother to ADHD 9yo... Please help me blend my family in a healthy and fair way

[harmonyj]

I have a 9 yo SS, he had just moved in with us in Nov 2007. My husband and I have been together for 6.5 years and Married for almost 4. I have been apart of SS life since he was 3. My husband and I have a 21mo. old son together and we are excepting again in may. My SS has ADHD and is a very sweet boy. He is not aggressive and tries very hard to be accepted. He has a very low self esteem and is easily manipulated to agree with you. But because he is ADHD he is out of control meaning that he can not sit still comprehend or focus. I am having problems as a new mother and adapting to my role as SS new SM. It feels that I am the only one on top of the game, trying to keep SS in line. When SS plays with DS. SS he does not realize that DS is 21mo and fights with him over toys tattles whines or plays with him too rough. And he does not understand that DS does not have the social skills that older kids have. In SS mind DS and himself are equal in everything. I am not trying to say that my 21mo is more important than SS but I watch other kids same age as SS play with DS and they are calm patient and do not fight over toys with him.
I do not know if this is a ADHD problem or a seeking attention problem. We are Hard on SS because of the ADHD he needs to be kept on Task. But SS told his mom we are way too hard on him and not nearly hard enough on DS. Mom did not like this.
I am at my wits end, I am trying to deal with this the best I know how but I just dont know any more. I am fearful of SS despising us because we treated him differently ( we have to because of the ADHD) or that my Marriage will fail because of the stress that is caused by this. I do not want it to be that way but if we do not figure out a way to make SS feel like an active part of the family who is EQUAL in love but differnt from our other children because of Maturity levels and age. I think something very negative will end up happening.
I love my family very much, I love my husband and children. I love my SS in a differnt way but care so much for his well being and future.
What can I do to save my family from negative recourse?

[PGB]

I have a soon to be 9 year old son w/ADHD myself.  They are quite the handful, no doubt.  But, with my son taking his meds like he is supposed to, we have very little trouble.  Has the SS been officially diagnosed with ADHD?  Properly tested for it?  If so, does he take medication for it?  I was COMPLETELY against the meds at first, as was my husband.  But, eventually we realized we were causing our son to miss out on a lot of great things that he will need...like social skills and a top notch education.  The difference in him has been amazing since starting the meds.  

But, even having ADHD doesn't excuse everything.  He is plenty old enough at the age of 9 to understand that while it's ok to play with his little brother he has to be more careful with him that he does with other children his own age.  Maybe sit him down and talk to him?  Explain how much you love him and how happy you are to have him living in your home, a part of your family.  And how much his brother loves him and likes to play with him, but that his brother is still a baby in a lot of ways and that he has to be the big brother and take care of him.  Make it seem like an important role for him to play.  Everyone, young and old alike, likes to feel needed and wanted.  Best of luck!! I know from personal experience that being a step mom is no easy task~