It's worth remembering that Danniella left the show before Barbara started playing Peggy. She came back for a few brief stints but Danniella's main stories were with Phil, Grant, Ricky and later Bianca. Barbara actually had more to do with Kim Medcalf's Sam than she did with Danniella's. It was Peggy who fetched Kim's Sam from Spain, it was Kim's Sam who Peggy came back to Walford for twice (once for her wedding to Andy Hunter) and it was Kim's Sam who Peggy fought to get released from prison when Chrissie framed her.
I don't know what Barbara's relationship with Danniella is currently like but based on Danniella's well documented behaviour in the 90's it probably wasn't great when they crossed paths since Barbara is known to be extremely professional. She fell out with Jessie Wallace over her bad behaviour in the early 2000's.
I do think it's strange the original Mitchell trio of Sam, Phil and Grant won't apparently be reunited on screen nor will the great Mitchell reunion of the 4 main Mitchells come to pass but it could be down to scheduling issues with the actors. Too bad DTC didn't plan the whole thing better considering he's had months to arrange everyhing.