J.,
The fact this woman knows she's terminal means there could be things happening right now that are helping her husband and daughters make peace with her passing, most especially if she's home and will die there and Hospice is involved. So if she doesn't want a wake, funeral, or even a memorial service that's her and her family's decision and unless invited to voice opinion, all others should be supportive and not get in the way. You do not need a service to say goodbye to someone or put closure on their death. Since these girls are aware their mom is dying and have been through whatever this cancer journey has taken her to from the start, it's not going to be a shock that she's gone and they very likely have been grieving quite awhile.
Death and grief are very, very personal and we all handle them differently. It seems you feel strongly about showing remembrance for this woman so maybe a contribution to the American Cancer Society, forming a Relay for Life team, getting involved in the Three Day or the Susan G Komen Foundation in her name is something you, personally, could do to show respect for her passing. As to the girls, just be supportive and available and let their dad deal with how their mother's passing should be memorialized for them. I'd also suggest cooking a meal, offering to clean house, or to take the girls for an afternoon. Those things could be done now too, and not just after their mom dies.
I think it's great you want to help the girls, but think about doing it within the parameters their mom and dad are setting up, and not what you feel is the right thing to do. If it were your children and the tables were turned, say you wanted a service but others around you felt you didn't need one and went about putting a stop to it, you wouldn't appreciate that anymore than she would appreciate you trying to do something with her girls, when she doesn't wish for it to happen.
S.