Verified answer

It depends on whose version of history you read. Cleopatra and her brother were co-rulers. It was very common within the Ptolemies to have co-rulers, or potential co-rulers killed. Cleopatra received help from Julius Caesar, but basically she already had the right to rule - he merely helped he rule alone.

By this time, Rome was very powerful,although still a democracy (in name at least), but Julius Caesar's political position was too tricky for him to really take over Egypt - far easier to have a friend in charge. It was later, during the war between Antony and Octavian (later Augustus) that Egypt became a full province of Rome.

Before Caesar came she held co-powder with her brother, but was being edged out. They had a civil war. Caesar put her on the throne. At the time Rome was the greatest power on earth. Yes, more than Egypt.