I’m another one who thinks this is a bad idea. I’ve been at a few funerals where things have gone wrong or tensions have been high in advance.
I remember being at one where there were tensions between the deceased’s partner (with whom he had 4 children) and his mother and siblings. The partner did tell them the time and date of the funeral, and the siblings turned up and tried to force themselves into carrying the coffin almost upending it, there was a massive brawl outside the church and the whole event ended up being anything other than a respectful remembrance of the deceased.
In the last few weeks of my own sister’s life, my brother was threatening to withhold the time and details of her funeral from her unmarried partner of nearly two decades and her closest friends with whom he had had a falling out. Even going so far at one point to say he would make sure there was no funeral at all and no one would be notified of her passing.
My father when he passed had left instructions that only my mother and his children could attend his funeral and that no one could come to the house between his death and the funeral. Those instructions were respected by the family but friends and more distant relations could well have been unhappy and thought this odd on our part.
A popular colleague at work died suddenly earlier this year. His family refused to give details of his funeral which they said was private and these were not published online anywhere nor was there any form of online remembrance book or death notice.
The reality is that the decision about how to mark a death is for the person tasked with making the arrangements. These aren’t open events (unless the family agrees to this - which many do, but this family haven’t) so if the person paying the undertaker does not want your step daughter there, there isn’t a way in which she can force herself in.
If she is internalising the reason for her exclusion as the argument with the new partner, she may need some help working through this emotionally