Sophie Friederike was very close to her parents and to her siblings. I have never had the impression that the children of Max I. made a real difference between siblings and half-siblings.
Sophie adored her mother Caroline. When she came to Vienna, she started to write very emotional letters to her beloved mother in which she informed Caroline about everything that happened at court and Vienna, but also about her own feelings, fears and opinions.These letters were much more initmate than Sophie's diaries she started to write after her mother's death in 1841.
Sophie was of course very close to her twin-sister Maria Anna, but she also wrote thousands of letters to her sisters. If you look at the sources in different books, you may find out that she wrote most of the letters to Auguste, Marie, Amalie and Ludovica. Maybe there hasn't been a historian yet who had a look at Queen Elise of Prussia's estate.
Sophie's relationship to her sister Charlotte - or Caroline Auguste - was a bit, let's say, tense. Charlotte was Empress of Austria which means she had the position Sophie had always longed for. After Emperor Franz I.'s death, Charlotte still lived with the imperial family and was quite popular with the Austrians and also at court. Sophie seemed to be a bit jelous and didn't want her (half-) sister to have such a strong position at court and even in family life. Charlotte realized that and retired. But this doesn't mean that they didn't get along well with each other. If you read the diary or the letters of young Franz Joseph you can see that he spent a lot of time with his "Großmama". Sometimes he even called her "Tante Großmama" (= aunt grandma).
When Ludwig I., who prefered his real siblings Auguste, Charlotte and Carl Theodor, had the well-known love affair to Lola Montez, Sophie was shocked and was also afraid that her brother would lose his throne (which later really happend).
Prince Carl was the favourite brother of almost every daughter of Max I. Especially Ludovica loved him a lot. Carl once said that there is a difference between his private and his public life. In his opinion it was nobody's business to know with whom he was in love. So Carl's morganatic marriages weren't that important. Sophie often met him at Tegernsee when she had already been married.
