Belinda,
My husband was adopted when he was 6 weeks old. He hasn't had any contact with his birth parents. I don't know too much about his adoption but what i do know is this...
His mother gave him up at birth due to her age (16) and no father support. I believe there was a six week "cooling off" period in case she changed her mind. After that he was officially adopted by his parents now. that was about 33 years ago.
she was able to get back in contact with him once he turned 18 but before that his record was sealed. she found him about 13 years ago and showed up at the door and his adopted parents ask her to leave (don't agree with that decision though) as he wasn't home at the time to make the decision himself. he's had no other contact from them.
I could never imagine the pain your sister is in. it would have been a very hard decision to make but she made it. maybe your sister could contact the agency that set up the adoption and ask to them make contact. perhaps she could be introduced as a family friend or something. my husband was told early on his was adopted but in many cases this is not done. in any case the child must be put 1st at all times.
In saying all this your sister chose to have more children..she should embrace them and love them as she knows what it feels like to lose someone. the other children should not suffer. maybe she also needs to talk to someone who can offer her professional advice...i know i've rambled on but i hope this helps you a little. just try to be the supporting sister always.
take care...