As I am sure you are all aware there is more than one Gostomysl recorded in the deep Russian history.
On Geni at the moment we have an MP,Gostomysl, Leader of the Ilmen Slavs which appears to be a meddling of the two men.
Gostomysl, son of Cedrag, can be read about here: https://ru.wikipedia.org/wiki/%D0%93%D0%BE%D1%81%D1%82%D0%BE%D0%BC%...
The other Gostomysl, father of Umila, can be read about here:
https://ru.wikipedia.org/wiki/%D0%93%D0%BE%D1%81%D1%82%D0%BE%D0%BC%...
or
https://en.wikipedia.org/wiki/Gostomysl
There is another relevant Geni profile: Gostomysl, Grand Prince of the Obrodites who is the son of Cedrag, Prince of the Obotrites which is a duplicate of the MP Cedrag, Prince of the Obotrites.
What I would like to hear about from users is the children who are currently connected to the MP profile of Gostomysl
Father of Mezislav Vendel of the Obodrites; Vybor Obodrite; Ivan "Eremite" of Czech Vendel of the Obodrites, Saint; Tabemysl of the Obodrites; Oldest Daughter Vendel of the Obodrites (Obodrite); Miloslava of the Obodrites and Umila, dau. of Gostomysl « less
Umila definitely belongs with the MP profile but according to wiki.ru Saint Ivan and Tabemysl belong to the other Gostomysl.
"Oldest Daughter" belongs to the MP as Umila was the middle daughter.
What of Mezislav, Vybor and Miloslava?
Miloslava of the Obodrites could conceivably be the third (youngest) daughter of the MP Gostomysl, so Umila's sister.
According to Tatishchev Gostomysl had 4 sons as well as his 3 daughters.
"Three daughters were married to neighboring princes of Gostomysl, and his four sons died during his lifetime."
Are Mezislav and Vybor two of these sons?