If it turns out that the marriage had been delayed to give this young woman time to process her sister's death and give her consent then Id view it differently. But, from the way it's been reported, that didn't happen - her sister's body was still in the house according to all those articles. That does not sound ok to me.
Likewise, if Diana had known about Charles's relationship with Camilla when she first agreed to marry him then no problem as far as I'm concerned. But according to all reports I've seen, by the time she found out, the marriage plans were in full swing and she felt unable to back out.