Johann Peter Keller's house Landstraße No. 51 in the Raaben Gasse
Although we have absolutely no documentation as to how Haydn made the acquaintance of his father-in-law, Pohl assumed that Haydn was introduced to the wigmaker's family by Keller's brother Georg Ignaz Keller, a musician at St. Stephen's, whom Haydn had known since his days as choirboy at the Cathedral. Georg Ignaz Keller was born around 1699 in the Bohemian town of Chlumec nad Cidlinou and came to Vienna before 1726 as an employee of the Bohemian Court Chancellor Leopold Count Kinsky, for whom he served as chamberlain and violinist. Therefore Haydn scholarship universally assumed that Haydn's father-in-law also hailed from Bohemia. In his 1956 article "Joseph Haydns Jugendliebe" ("Joseph Haydn's Early Love") Ernst Fritz Schmid vividly describes Georg Ignaz Keller's progress as a musician in Vienna and how he rose from a simple servant to a violinist at St. Stephen's Cathedral in 1731 and a court musician in 1765. In spite of complete lack of evidence Schmid presents the kinship between the "Keller brothers" as fact:
Der kaiserliche Hofmusikus Georg Ignaz Keller ist es nun gewesen, der Haydn die Bekanntschaft mit der Familie seiner Jugendliebe und damit auch seiner späteren Frau vermittelte. Kellers älterer Bruder, der "hofbefreite" Perückenmacher Johann Peter Keller, der um 1691 ebenfalls in Chlumetz in Böhmen geboren war, besaß zu Wien in der Vorstadt Landstraße in der Ungargasse ein eigenes Haus und einigen Wohlstand [...] Georg Ignaz Keller brachte Haydn in das Haus des Bruders, wo mehrere Kinder, darunter anmutige Töchter heranwuchsen, deren Klavierunterricht der junge Meister übernahm.
It was the imperial court musician Georg Ignaz Keller, who established Haydn's acquaintance with the family of his early love and also of his future wife. Keller's older brother, the wigmaker to the court Johann Peter Keller, who had been born around 1691 also in Chlumetz in Bohemia, was considerably wealthy and owned a house in Vienna in the suburb of Landstraße in the Ungargasse [...] Georg Ignaz Keller brought Haydn into his brother's house, where several children were growing up, among them lovely daughters whose piano lessons were taken over by Haydn.
The 1722 marriage entry of Haydn's parents-in-law: "Dominus Joannes Petrus Keller, Joannis Georgij, et Aloysiæ filius, cum Virg:[ine] Maria Elisabetha Seillerin, Georgij et Elisabethae filia, Tes:[tes] Do[min]us Ferdinandus Marher, et D:[ominus] Antonius Geissnhoff. 12. [November]" (A-Wstm, Tom. D, pag. 345).
Den 31 October 1722 copulati sunt 12 9ber 1722 / Der Kunstreiche Herr Johann Peter Keller, ein Keÿ[serlich] Hofbefreiter Porakhenmacher,
bin dem / 3 tauben in der unde[r]n Preinerstrasen wonhaft. / Zu Hamburg gebirtig, des H[errn] Johann Georg Keller, undt Frau Aloysiæ sel[ig] beder / ehlicher Sohnn. nimbt zur ehe die tugent / same Jungfrau Mariam Elisabetham / Seillerin, des H[errn] Georgij Seiller und Elisabethæ sel beeder eheliche tochtor / Zu Wacheram in Österreich gebürtig, / beÿ den 3 taube[n] in der under / Preinerstrass wonhaft. / 1 2 3 Ambo in parochia / per plures annos. (A-Wstm, Verkündbuch 1722, 31 October 1722)
And there we have it: a Haydn-trifle of world-shattering insignificance. Haydn's father-in-law, the wigmaker Johann Peter Keller and the musician Georg Ignaz Keller were not brothers, but came from quite different regions in Europe. It remains to be investigated if Georg Ignaz Keller played any role at all in Haydn's life. Of course one could argue that in the above entry the priest actually referred to the Austrian town of Hainburg. But the fact that the word definitely has an "m" and only Wagram bears the attribute "in Österreich" (as if to set it apart from the non-Austrian birthplace of the groom) makes it very likely that (apart from Brahms having written variations on a tune not by Haydn) the hansa town has finally gained a family relationship to the composer of the German national anthem.
Approaching the end of his life Johann Peter Keller suffered hard times, which in a fascinating way may reflect the demise of the wig as a social and economic factor in 18th-century Vienna. He died on 9 August 1771, absolutely destitute in the "Klerfisches Haus" on the Hoher Markt and was buried the following day in the new crypt of St. Stephen's. His relatively wealthy children (and maybe his son-in-law) made sure that his 3rd-class funeral cost 27 gulden 36 kreuzer and even comprised a "Music vors Miserere" for 4 gulden.
The entry concerning Johann Peter Keller's exequies at St. Stephen's on 10 August 1771 (A-Wd, BLB 1771, fol. 227r).