Mirza Sultan Ahmad was the eldest son of MGA, he never accepted the prophethood or even the “imamate” of his father, however, nor did he ever reveal any of the family secrets.  I have written about him here:

The Qadiani Khalifa did a fake conversion when Mirza Sultan Ahmad died
As we all know, the Qadiani jamaat engages in lots of fake news propaganda, in this case, they made it look like Mirza Sultan Ahmad did a bait at the hands of the Khalifa while on his death-bed.  Its a total lie.

Mirza Sultan Ahmad seemed to support the Lahori-Ahmadis in 1916

In Paigham Sulh, 23 January 1916, there is a lengthy, 3-page article entitled Milad Muhammad (saw) by Khan Bahadar Mirza Sultan Ahmad, who was the eldest son of Hazrat Mirza Ghulam Ahmad. On the first page, last column, I have marked more than a half of the column by a red line in the margin where he deals with the finality of prophethood. To quote from it:

“The second task of the Holy Prophet was Khatm-i nubuwwat. Before the Arabian Prophet, not even one prophet who came claimed khatm-i nubuwwat with the emphasis that our Hazrat (saw) did. The effect of that is that till this day no one in the entire world dared to be a claimant to prophethood after that Khatm-i nubuwwat. … Other religions did not enter into this at all, while within the Islamic world, in fulfilment of [the hadith] ‘The Ulama of my Umma are like the prophets of the Israelites’, such persons kept on arising who in their own sense possessed the glory of Israelite prophets. No less were the Islamic mujaddids in their blessings. Thousands of such venerable ones have there been in the Umma…

All the prophets gave good news of the coming of our Prophet, and our Prophet gave the good news of hundreds among his own followers who arose in Islam as abdal, aqtab, auliya [saints] and mujaddids. … Prophethood was ended and spiritual benefits were made general in another form.”

See here for the Urdu version