congratulations on your pregnancies :)
DD1 (3.5) has severe s&l delay + autistic traits. DD2 (so far she appears NT) is 10 month old.
we were very worried how DD1 would react to a sibling but she really amazed us. she loved her sister from day one and DD2 is just "teaching" so many things to DD1. it is through DD2 that DD1 learns how to share, how to care, to show compassion: DD1 isn't usually very social and caring but for some strange reason, she makes sure that DD2 has a blanket on her, she conforts her when she cries, she brings DD2 her water, loves to feed her, gives her hugs and kisses (DD1 otherwise never ever kisses or hugs people without being asked to), etc, etc. as DD2 is still only a baby, I am sure we are on a wonderful journey together.
piglet, the sleep issue will probably boil down to DC2's sleeping habits. DD1 was a terrible sleeper as a baby, sometimes screeching throughout the entire night. but we were very lucky with DD2. excellent sleeper from day one. she sleeps in a cot beside our bed (well, most nights we end up co-sleeping) and only whines a bit if she wants a BF. in that sense, having DD2 didn't make any difference to DD1's sleep. if anybody is getting less sleep, then it is me 