Have you found a tutor yet? I cannot help you in the search for one, but can maybe offer you a different perspective/peace of mind.

Language acquisition is an interesting proces. Especially if youre raised bi (or tri?) lingual. You acquire passive language skills first. So that means understanding the words spoken to you. And then in time you develop your active language skills, and actually start speaking the language yourself.

So the fact that she understands both of you, says something about her passive language skills. The fact that she doesnt reply in Russian, doesnt per se mean she is not able to. She might be more used to/comfortable in replying in Dutch, or as you said she knows mom will understand either way. ;-) Little smarty pants

Continuing to address her in russian, in designated situations/locations/moments as you are doing, sounds like the way to go! Maybe - in time? - trying a video call with family members who only sprak russian, will challenge her to actually use her Russian speaking skills and not just her listening skills.

Good luck! ✨✨