Alana Mian, four, and Hameeda Begum, 71, died after a bin was set alight outside Mrs Begum’s Bolton home on 23 June 2008.
Three people have been sentenced for witness intimidation in connection with the case, but no-one has been charged with causing the blaze.
Martin Bottomley of GMP’s Cold Case Unit said: “Today marks 12 years since the fatal arson attack which tragically resulted in the deaths of Hameeda and Alana Begum.
“This very sad case still resonates with the local community and we will continue to investigate any intelligence or lead which could lead us to convict someone of these horrendous crimes. We have not given up hope and remain determined as ever to find their killer and provide the family with the justice they deserve.
“We will never be able to reunite this family with their lost ones, but by finding those responsible we will hopefully be able to offer this suffering family a small amount of solace by providing answers to their questions which have remained unanswered for far too long.
“If anyone at all has any information that could help lead us to those responsible, please come forward and speak to police. I understand that this event took place 12 years ago, but I believe that there are people in the community who may have valuable information that can assist our investigation.
“The £50,000 reward for information leading to the arrest and conviction of those responsible remains in place and we are aware that allegiances change over time. We will not close this case until those responsible are brought to justice.”