Usualy a men start his avelut for his father after burial.
If one of the son is not present on the site of burial, or if is in another state, he strat after the news. My question if one of sons is on fly trip, started before the burial and will land only in the nigt after the burial, when he start the shivà? His brother start in in the afternoon of wednesday, but he have the notice only in the night so is thursday, before start the trip he know that in the afternoon will burial hi father, but he cannot know if realy happen or not, sometime for bureaucratic problems may slip to next day… When start the brother in trip?
If he began the Aveilus during his travel, even if it was based on a doubt if his father was buried, it would count as the first day, if it turns out that the father was already buried at that time.
Pischei Tshuva Y:D 375:1
sorry for the delayed response.