So sorry for you Syd and your sister, but I'm sure you realise that the child's safety and welfare should come before anything else. One of the problems for mothers in your sister's situation is that SSD do not have the time (nor probably the inclination) to offer any on going support to your sister. Don't think you said how old the child is - this will be dependent on how quickly he will be adopted. The older the child the harder it is to find an adoptive family, so he will remain with his foster carers until an adoptive family is found. Presumably your sister is still having contact, but that will be cut down quite drastically now that the court have approved the adoption plan.
All prospective adoptors have training groups and he difficulties of birth parents is explained and they are encouraged not to judge the birth parents or to say anything negative to the child. Your sister will probably have the opportunity to meet the adoptive parents in a neutral setting - these meetings are very tense for both the birth parents and the adoptors, but adoptors are usually glad to have met the birth parent, though I'm not sure this is true for many birth parents, and some refuse the meeting.
I know it is of small consolation, but there is usually a system called "the letter box" organised by SSD so that your sister can receive an annual photograph via the SSD letter box and some adoptors send news of the child too, but this does depend of course on the adoptive families. Your sister can send cards/letters etc via the system, but it will be the adoptors who will decide whether to pass these on, dependent upon the child's age and understanding.
All adoptive parents have to agree to tell the child the truth i.e. that they are adopted, and provide a life story book, containing pictures of their "first mummy and daddy" and other "first" relatives, granny, cousins etc. It will also contain pictures of the foster carers etc so that the child has some understanding if his beginning.
You sound like a lovely sister and I am sure you will be a big support to your sister in the loss of her child.
Do take care of yourself too.