DD2 was a super cruisy baby so sometimes it was like she wasn't even here! She just slotted in so well to the way our family was at the time. I didn't notice any problems with the way they got on - they are all 20 months apart and they played together well. Of course there were times when they would fight and it would be 2 against one etc, but that's just a part of life - if you have 2 kids they can fight with each other too kwim? When she was first born though they ignored her really - DS1 wasn't interested in babies and DD1 was too little to be too worried about her too. It wouldn't have been till DD2 was 6mths+ that they really started to develop their relationships with each other.