A document from 1550 for a woman named "Osztrosics Margaretha de Giletincz," states she was the wife of "Egregii Joannis de Izdencz" and her brother was the late "Joannis Osztrosics." She owned property in Ljeszkovecz in Trencsén county, which is present-day Lieskovec, Slovakia.

NOTE: This reflects that the late brothers János and Miklós Ostrosith had a sister, Margit Ostrosith, wife of János Izdenczy.

SOURCE: https://www.familysearch.org/ark:/61903/3:1:3QHV-V36Y-ZPLL

On 21 Jun 1551, it is recorded in the Gedenkbücher record series stored in Vienna, that Blasy de Waradinopetri received 100 florins, as heir of Miklós Ostrosith. The translated document reads as the following:

 "We are sending to you with these presents the enclosed petition of our faithful servant, the venerable Blasy de Waradinopetri, Provost of Szepes, Councillor, as well as of the heir of the late Nicholai Ostrosyth. From it you will readily learn that we are being most humbly petitioned to pay the sum of one hundred florins, for which they are bound to George Horváth, and which money they borrowed for our affairs and for the pay of the cavalry who formerly served under the late Nicolao Ostrasith. Since the matter stands thus, we judge it fair that this debt be paid by us. Therefore we entrust the business to you, that if the facts are as stated, you pay to them the aforesaid sum of money out of the revenues of our Hungarian Chamber under your administration. But inform us nonetheless for what uses our money has been expended, and to which horsemen payment has been made from it, and in whose name such money has been assigned, so that our will in this matter may be duly carried out. Given at Vienna, the twenty-fifth day of June, in the year 1551.
Hungarian Chamber.

NOTE: This document makes it very clear that Blasy de Waradinopetri was an heir of Miklós Ostrosith, indicating a familial connection; presumably through a maternal line.

NOTE: The giletinczi Ostrosith, or Ostrosith de Giletincz, were a family of Croatian origin, then residing in Giletincz, Pozsega County in Slavonia. The progenitors of the family were siblings János, Miklós [mentioned in the document above] and Margit.

Miklós Ostrosith himself was a captain in one of Ferdinand I's armies and in 1536 helped liberate the castle of Makovicza, with help from Bártfa. In 1542, he took part in the Siege of Pest, and it seems he fell there, because Latin poems already printed in 1544 stated he was killed by a rifle bullet during the war against the Turks.

SOURCE: https://archives.hungaricana.hu/en/lear/BecsSeg/158129/
SOURCE: https://www.arcanum.com/en/online-kiadvanyok/Nagyivan-nagy-ivan-magyarorszag-csaladai-1/nyolcadik-kotet-6DB3/ostrosith-csalad-giletinczi-baro-7840/
SOURCE: https://hu.wikipedia.org/wiki/Ostrosith_csal%C3%A1d