Looks like there is a lot of jealousy going on which has not been resolved for a long time.
I suggest reading [Siblings Without Rivalry][1]. It addresses different scenarios and gives suggestion on what to do in different situation, including the ones you described above.
Meanwhile I suggest: 

 1. not to react to your daughter complains the way your husband does   
    (and this is discussed in the book as well). She might have figured 
    out that it gets her brother in trouble and use it. If not now, then
    in the future. if you did not see what happened you can't assign the
    blame. If you daughter complains tell kids to resolve the issue
    themselves. If it does not get resolved both would face
    consequences.
 2. Talk to you son and spend some time alone with him. Tell him you love him. A lot. Do the same with your daughter. But not at the same time.
 3. Don't put kids in the roles. Older brother does not mean more responsible and protector. Younger sister does not mean weak and blameless. He does not have to love his sister (though this is something we want as parents), but he has to treat her with respect.







 


  [1]: http://www.goodreads.com/book/show/12816644-siblings-without-rivalry