You are right, the word Dynasty means a royal house or a sequence of
rulers from a royal house.
The Egyptian dynasties were just that, the successive families that
inherited and ruled Egypt.
The exact lineage of the pharaohs in come cases, remains unknown.
However, most married within their families to keep the bloodlines
pure. And for the cases where there is lineage doubt, it is doubt
based on immediate parentage rather than doubt regarding the family
from which the king came.
If fact, genetic testing of the royal mummies is strengthening the family ties.
There are a few instances where non-relatives are included in a
dynastic list, such as the kingship of Horemheb which brings the great
18th dynasty to a close. But even General Horemheb knew the
importance of dynastic family ties.
His queen was Mutnodjmet. When she died we can deduce that she was
between 25 and 30 when the, by then, 5o-year-old General Horemheb
chose to marry her. This was of course a classic marriage of
convenience, enabling Horemheb to join the ruling family of the 18th
dynasty to secure the throne for himself.
Some later dynasties were not Egyptian to start with and the term
'dynasty' is merely a label of convenience, such as in the Persian era
of Egyptian Rule.
The Egyptian ruling families themselves are all contained within
twenty-seven genealogical trees, and as mentioned, with few
exceptions, they all run true.