So let me preface this by saying that I can not conclusively prove or disprove anything here. Since it presumably involves people of significant power normal rules may not have applied. However, there are a few inconsistencies with this story and the claims within it that are rather apparent if you're familiar with German aristocracy, the effect the reformation had on German monasteries and the area in general. So, let's dig into it:

First of all, the place of birth is Strietfeld, it's actually in Mecklenburg-Schwerin, it's a small hamlet in the very south of the Duchy, barely big enough to be on a map but it is close to Malchow. It is some distance (about 90 kms) from Schwerin and in the domain of the von Flotow family.

The von Flotow family was never particularly large or powerful in Germany but they did at times hold positions of some prominance in Germany. They are not however a royal family. But there is another family that is. The Duchal family of Mecklenburg-Schwerin. The head of the family is traditionally styled Royal Highness. So, if we are to Royal part of the claim, this is the family that would have had to be responsible for an illegitimate child. If not, the most likely lineage would be the von Flotows based on proximity but really it could technically have been any of a thousand noble families based on their habit of travelling for any number of reasons.

Based on this we can also exclude something else and that is the mother being of noble birth. Having a mistress was not uncommon for nobles at the time. Paul Frederick, Grand Duke of Mecklenburg-Schwerin for example lived much of his life with his mistress, Countess Catarina Hauke. Since she was of noble birth this was no big affair. He had six children with his wife and four with his mistress around the same time. Incidently making him a somewhat unlikely suspect as he prefered to spend most of his time in solitude with his mistress around the time your great grandmother would have been sired. Another possibility would have been an illegitimate child of George I of Mecklenburg Schwerin although the man would have been in his mid 50's at the time and was not known to be prone to such misadventures.

So, thus far, based on geography and genealogy I think we can exclude the claim to royal lineage, at least locally. There just aren't any good matches in the royal family of Mecklenburg-Schwerin. A local lesser noble or visitor to the von Flotows is of course entirely possible and nigh impossible to exclude.

So, let's look at her supposed upbringing, the monastery of Malchow. Now, this I find somewhat unlikely because following the reformation the Monasteries of Malchow, Dobbertin and Ribnitz were all converted into a Landskloster. I don't know exactly how to translate that. "State monastery" perhaps, state in this case refering to Mecklenburg-Schwerin. Now, these were exclusively for the daughters of the "landed nobility of good christian stock (meaning no catholics) and virginity". This was not the place you left an illegitimate child, the nuns generally have been in their 40-50's or older based simply on the considerable wait time for a spot at either monastery. Noble families would typically sign their daughters up for one of the three at birth in case they were to need a spot 50 years down the line. It's possible that an exception would have been made for a child if the right person of sufficient status asked but it is not likely.

Regarding her adoption, "striefela" is almost certainly a mistranslation or typo of Striedfeld as it is neither a place, not a word in German meaning that she was likely adopted by someone from the village in which she was born. Possibly her mother after she had married or a relative. Similarly, Warzen is almost certainly a mistranslation of Waren, a town about 20 kms from Strietfeld. Warzen means "warts" in German and is not a place.

So what does all of this tell us and under what circumstances would an illegitimate child have placed in the care of nuns in 19th century Germany? Well, a child could certainly have been placed in the care of nuns provided the family asking had sufficient power but it would be unconventional to say the least. Family chronicles are full of bastards and generally they were not left in monasteries, particularly not prestigeous ones like Malchow.

However, there is a possibility here that comes to mind, and that is that of her story being largely true. I will however say that this part is purely speculative. It may have been the case that she was placed in the care of the nuns for a period which allowed the mother to be wed free of the burden of an illegitimate child in the eyes of society. Supposedly the groom would have known of the arrangement. It is also possible that the unnamed father saw emigration as a solution to this situation and supported it financially. The monastery may have been an alternative to the child being placed in the care of the municipal youth office orphanage as I believe single mothers could not gain custody of children prior to 1950 or so.