Before David replies, a few remarks:
"Prince" and "kniaz" are synonyms, while "tzarevich" means "prince of blood", or "royal prince". After the death of Tsarevich Alexey, Peter I's son, the latter title was used only by descendants of former Caucasian and Tartar ruling dynasties. The Sibirsky family was degraded from "tzarevich" to "kniaz" in 1718, following its involvement in the Tsarevich Alexey's assumed conspiracy.
The Russian Biography Disctionary mentions two Princes Sibirsky:
- Vasiliy Feodorovich (grandson of Vasiliy Alexeevich), full general and later senator, b. 1761, d. ~1809;
- Alexander Alexandrovich (grandson of Vasiliy Feodorovich), renown archeologist and historian, b. 1828, d. 1879.
I very much doubt that former Muslims converted to Christianity were ever allowed to return to the Muslim faith. In fact, such an apostasy was considered a capital offense by Russian law.