Thanks for the explanation. That means the current Jacobite line is not questioned in that regard.
It does leave other issues with Jacobite pretenders. Franz is smart enough not to refer to it at all. If only the titular duke of Parma and Piacenza did the same when it comes to Carlism.
Lastly was there not a marriage between an uncle and niece in the line of Franz? I believe such marriages were illegal in England, in that case would the offspring still be legitimate when it comes to English law?
On June 20, 1812, in Cagliari, on the island of Sardinia, Archduke Franz of Austria-Este (later Duke Francesco IV of Modena) married his sister's daughter Princess Maria Beatrice of Savoy. Princess Maria Beatrice was the eldest daughter of King Vittorio Emanuele I of Sardinia, and the heiress presumptive to his rights to the English and Scottish crowns. Francesco IV and Maria Beatrice are the great-great-great-grandparents of Franz Herzog von Bayern.
The 1812 marriage in Sardinia was legal in Sardinia, since Pope Pius VII had given a dispensation for it (and it was recognised by the bride's father who was King of Sardinia).
If an uncle and niece had attempted to contract marriage in 1812 in England, such a union would have been null since it would have been in contravention of the 1662 Book of Common Prayer's Table of Kindred and Affinity.
The marriage of Franz and Maria Beatrice did not take place in England. Being contacted in Sardinia and being legal in Sardinia, it was recognised as being legal in England.
This is not just a matter of Jacobite opinion (and Jacobites by their very nature tend to be rather legalistic about such matters). There are comparable cases, and one of them involves the succession to an English title.
In 1800 in Palermo (in the Kingdom of Naples) Sir John Acton, 6th Baronet of Aldenham (and also Prime Minister of the Kingdom of Naples), married his brother's daughter Mary Ann Acton. They had three children. In 1811 Sir John died and was succeeded as Baronet by his elder son Ferdinand. This Ferdinand in turn was succeeded as Baronet by his only son John, the famous historian, who in 1869 received a UKGBI peerage as "Baron Acton".
A marriage legally contracted in the Kingdom of Sardinia or in the Kingdom of Naples is also legal in the Kingdom of England - even if such a union would not have been legal if it had been contracted in England.