Was the Viennese Publisher, Joseph Shlesinger, the brother in law of Rabbi Shimon Sofer, author of Michtav Sofer?

In 1898, Joseph Shlesinger, the famed publisher of Vienna, bought out the rights of the Sefer Pele Yoetz, by Rabbi Eliezer Papo, from Papo's son, Yehuda Papo of Jerusalem. Following the title page is an announcement by Rabbi Yehuda Papo that the rights were sold to Shlesinger, and following that page is a letter printed, by Rabbi Shimon Sofer, banning anyone from republishing the Pele Yoetz for 5 years.

Letter of Shimon Sofer

Rabbi Shimon Sofer is introduced as "my brother in law", though I was not able to find any record of any such relation elsewhere.

Turns out though, that the ban hadn't much effect, Bet Eked Sefarim lists several editions within the 5 years after this 1898 printing.

Update: 1/24/14
a friend, who is a descendant of the Chatam Sofer has supplied me with the following information:
Reb Akiva Eiger's daughter Sorel was married in  תקס"ג to Reb Avrom Moshe Kalisher . and was niftar in תקע"ב they had two daughters. They both came along when she married the Chasam Sofer. one died r"l at a young age, the other one got married to Reb Yosef (Ginz) Schlesinger, who was a talmid of the Chasam Sofer too.