The genealogy in the Icelandic sagas is just as reliable as any other secondary source. The sagas where written down 100-400 years after they happened. The stories where handed down orally before that. There are evidence of some earlier written versions of the stories that didn't survive.
But, comparing different written eksempels of the same stories has shown that they are fairly consistent.
But, as with every source, you have to consider the origin. These are a secondary source and should be treated as such.