Complex sentence: This expectation was fulfilled within two months of the birth of their first-born child: on 5 October 1740, the infant Ivan was adopted by his grandaunt (who was on her deathbed) and declared her heir apparent.
Split into simpler sentences: This expectation was fulfilled within two months of the birth of their first-born child. On 5 October 1740 the infant Ivan was adopted by his grandaunt (who was on her deathbed) and declared her heir apparent.