Neither my sister nor I need / want to have a service. Will others be disappointed?

Many people may, people grieve at different times in different ways, a memorial is for the living to give respect to the departed.

It is up to you, but it may help others, and you may find after it is over it may help you also

When people said that the funeral was more for the living than for the dead, they implied that dead people had no more feeling, only the living could argue, feel happy, sad, angry, etc.
You are the living, do what make you feel at peace while living.

Do what works for you and your family, not for acquaintances. Sometimes there is closure in a memorial service and that leads to peace. Sometimes it just reopens the wound.

What would your mother have wanted?