What does her father, your ex, say about this? I suggest he is the one to tell the step mother to stop including your daughter. You need to discuss how you feel about this and he needs to ask his wife to stop.
Seeing a counselor together is a good idea if everyone will go. If they won't you go to get help in dealing with this situation. You may have to be the one who discusses this with her. You don't need to hurt her feelings just be honest about how you feel. The way you wrote about it here is OK.
I agree that your daughter should not be involved in this. And if the death was years ago, grieving should not be so intense. Does she live with them? If the mom is talking about this all the time then it's not healthy for her own daughter.
If your daughter does not live with them you can talk with her about how this isn't her daddy and about your view of the situation without making it an all the time conversation. I would talk with my ex and tell him this situation is not healthy for the girls and enlist his help in finding a healthier way to handle this.
I looked at your previous posts. This has been going on for years. If the stepmom is still treating his death as if it happened recently I suggest that this could be a serious issue for your daughter. How is she handling it? She may have learned to ignore it. Or she could be having emotional issues of her own. If so, I would have her talk with a therapist. And if the step-mom's grief is causing your daughter difficulties I would consider finding a way to change the time she spends with her. Perhaps your ex would consider changing the parenting time arrangements if you were able to show that his wife's grief is negatively affecting his daughter.
I think that a counseling sessions with all of you is a good idea. The step-mom needs some help. If she'd be willing to go, hospitals have grief groups that could be helpful. They are free and open to anyone, not just for someone who has used the hospital.