I am so sorry for your loss. How tragic for all of you. I hope your moms recovery goes well.
Did they have a special place in town that they liked to visit? Maybe a park, or pond? You could all go and release just a small amount of the ashes at the end of a gathering with all of you there. Place them in a small container.
Maybe say a few words. Then like Sarah says, let whoever wants to say anything say it, share it sing it. You could also hire a student musician to attend and play an acoustical guitar as people arrive and leave.
The balloon release is done all of of the time by Hospice for grieving children. It is done where each child says or thinks of their loved one and then releases their balloon (rubber) . It is a visual for them to see it fly.
At one of the memorials for my stepmom's daughter they released a "dove" (actually a white homing pigeon) . There is a company here in town that rents them. You will receive it in a box on the day of the memorial.. They all said her full name opened the box and the dove flew up and back to its home.. It was very touching..
Just make the memorial what YOUR mother wants and needs. Others will understand.