When reading this Wikipedia article I come to the conclution there is only 1 Frosti;
http://en.wikipedia.org/wiki/Fornj%C3%B3t
It is however a bit confusing - see the chapter about "More traditions about persons named Frosti and Logi".
Quote;
"In the Ynglinga saga the names Logi and Frosti are otherwise connected when it relates that King Agni of Sweden in a raid on Finland killed Frosti, the leader of the Finns who opposed him and captured Skjálf, Frosti's daughter, and her brother Logi. (But the verse of the Ynglingtal quoted here as confirmation says only that Skjálf is Logi's kin.) "
The word "kin" is here meant as a descendant of someone, several generations later. This must be so also, since the "1st Frosti" is from around year 250 and Skjálf from appr. year 428.
The Family Tree at the bottom of the page show the relationships easily understandable.
According to this article Skjálf is the "wife of" Agne, the son of Dag the Wise and should probably be connected to the tree in that way only.
What do you think - does this make sence?