When the cartel were dead and buried in the middle of season four the show needed a new power dynamic, lasting into season 5. With an interesting back story and love interests for both Richard and Emma the Rossinis' certainly provided that. Fr. Christopher was also part of the family, much to Anna's resentment because of her husband Dominic's affair with Julia.
To sum up the Rossini's input:
They achieved what the other opponents of Angela (the cartel, Jacqueline and the 13) were not successful in doing i.e. obtaining a controlling interest in the wine business. Melissa Agretti and Michael Sharpe also got one over the grape granny but both of them owned the whole shoot and bang. Therefore Angela and in the case of season 9 her family were outside the business, with no say at all. A split in any business will undoubtedly create the power struggles the 80's prime timers were noted for. As for the Rossini estate fire it was blamed on Angela but before Anna exited it was discovered she was a pyromaniac (a disorder in which the individual can't resist the temptation to start fires).
Anna Rossini, her daughter Cassandra and their accountant are visiting Angela, Chao-Li and Greg to look over the Falcon Crest ledgers. The Rossinis with their majority share of the wine company have a plan to build a resort on the land where the winery sits and the 500 acres of vineyards, (a scene from season 5, 1985 - '86):
The meeting takes place in Angela's study.
Angela to Anna: "Well what can I do for you?"
Anna: "First of all I want a comfortable chair. This one is a disaster."
Angela: "Your court order says you can look at my ledgers. It doesn't say I have to make you comfortable."
Anna: "I think you ought to know I'm about to move into that little guest cottage, up near the winery. I think I can oversee things better from there."
Angela: "Will you now. No one has stayed in that cottage for years. Watch out for the rats!"
Angela then walks away from Anna and goes to her desk where Cassandra is looking at her new business renderings. Mrs. Channing says that the resort will be an instant slum and that it will over her dead body before the vineyards are ploughed through.