we found that giving ds1 a baby of his own (a baby born doll) seemed to help cos he carries it around and even tries to breastfeed it! has she got a dolly of some sort?
i try to get ds1 to get me to help out with ds2 as much as possible- he loves helping out and looking after his little bro. we bath them together and ds1 washes ds2's belly and he helps with changing his nappy. i think this has stopped the jealousy a little.
it's a hard time, but just remember, you have given your ds1 the greatest gift there is- a sibling.