do I meet my niece & nephews adoptive parents??

So 15 months ago my sister put my 5 year old niece and 1 year old nephew into care, she did nothing to help herself get them back etc. Basically she did it all because she has a new bf and wanted the freedom, she went off the rails into drugs alsorts. 
She did everything she cud to stop me and my partner from having the children including making up lies saying my bf assaulted her which was a lie and police found out she was lying.
Anyway we've not be allowed to see the children and found out yesterday that they are being adopted at the end of the month 😭. Well I've have nothing to do with me sister since she sent death threats to me and my partner and threatened to kill us with my son in the house. Yesterday the social worker told me that my sister has refused to meet the adoptive parents and she's asked if I would like to meet them.
I really torn as these people will be taking care of the children and moving them away from the area. My niece will have to start a new school she's not going to know anyone. It's going to be really difficult for them both. So I'm unsure if I should meet them, if I do what do I say to them?
Update:-
Thank you for all your comments I love these children like they are my own, I look ed after them more than there mom did. I have had highly praise from the social worker of how the children have been brought up and she said all this will be in there paperwork for wen there older. I'm going to write a letter and give them a tardy each plus a memory book and as many photos as possible. The social worker has asked for all their childhood photos as the children will want this growing up
UPDATED again 
I am meeting the adoptive parent next week they are having a closed adoption but they are willing to send me a letter each year and I can send them a letter back. It's going to be such an emotional rollercoaster but I feel I need to do this for them. Thank you for all your kind words xxxx