oh hon, don't feel bad. it's to your credit that you want to be involved, but it's a difficult situation.
i agree with maryz, you would not have been considered a good candidate to take on their care due simply to the family relationship, and without having all the facts, it sounds like being placed with a loving adoptive family is the best thing for these kids.
when they are adults, they may have questions for you, like health history. it would be nice to be able to get that information from someone like you where there isn't the difficult emotional baggage they will likely have with their parents. let them determine what their contact needs will be when they are old enough, are emotionally ready and can make a decision to initiate contact on their own terms as an adult.
as an adopted person, i can say, it is nice to know someone thinks of you and would be willing to talk with you if you wanted to, even if you don't take them up on their offer or wait quite a while to do it. when i met my birth family, i was really touched by how my grandparents filled that role for me and it really helped me. the actual meeting with the birth parents and subsequent relationship has been pretty much agony and misery, so it was great to have some more neutral adults i could talk to.
right now though, they are building crucial bonds with their new family, and that process needs time.