Perhaps your daughter,
depending upon her age,
doesn't need to visit her dad's house for a while.
At least until she's more at peace with her loss.

What's her dad say about this mess at his house?
He ought to be looking out for his daughter's best interest.
That includes allowing her to grieve for her sister.
And he needs to understand that fact.

As for you and the step mom....
if you don't get along, then don't try to during this difficult time.
Deal with your ex - the father of your child.
It's who you should be dealing with about this issue anyway.
She is your shared concern.
And the step mom doesn't seem to "get it" anyway,
So, let her be.

These are just my own thoughts.
I could be all wrong.
Wishing you easier times ahead...

Peace!