There were no male Romanovs in 1730-1740
Yeah, you've really got to ASB this to even get a Romanov for her to marry. Peter II was the right age, but he died young; even if he lived, the other great European powers wouldn't have stood for the unification of the Russian-Hapsburg thrones.
Here's a possible scenario though:
1708: Charles VI marries Elisabeth-Christine of Brunswick-Wolfenbüttel
1711: Charlotte of Brunswick-Wolfenbüttel marries the Tsarevitch Aleksei Petrovitch.
1715: Peter Alekseivitch is born to Charlotte and Aleksei. His mother dies shortly after.
1716: Leopold, first child of Charles VI and Elisabeth-Christine of Brunswick-Wolfenbüttel, is born. POD: he survives infancy.
1717: Maria Theresa, daughter of Charles VI and Elisabeth-Christine, is born.
1718: Aleksei Petrovitch dies.
1725: Peter I dies. POD: On his deathbed, he names his ten-year-old grandson Peter Alekseivitch as his successor.
1727: Death of Empress Catherine I of Russia. Peter II proclaimed emperor, is backed with more support than in OTL due to the recognition of his grandfather. Immediately negotiations begin with an eye to a suitable empress for him.
1728: Maria Theresa of Austria is tapped as the most likely candidate for Peter II. Her mother Elisabeth-Christine promotes the match, as Peter is her own sister's child. It is expected that Maria Theresa will make Peter II more sympathetic to the Catholic/Hapsburg cause. Peter II for his part is tickled by the idea of having such a blue-blooded bride.
1730: Maria Theresa is dispatched to Russia to marry her cousin Peter II. Her elder brother Leopold is expected to succeed as their father's heir in Austria-Hungary. POD: Peter II does not die young but instead marries Maria Theresa.
1731: Marriage of Maria Theresa to Peter II.
1735: Birth of *Aleksei, first child of Peter II and Maria Theresa.
1739: Sudden death of the Archduke Leopold, heir to Charles VI. A succession crisis ensues. Tsar Peter II makes it clear in letters to his father-in-law that he considers his own son, Aleksei, the natural successor to Charles VI.
1740: Death of Charles VI. A new War of the Austrian Succession ensues as Peter II presses his five-year-old son's claim to the Hapsburg empire.