I am currently very interested in Sumerian mythology and would like to find out more about it. However, I am finding it very hard to keep track of the gods lineage and parentage as well as their descendants.
To keep track of them I would like to have a family tree of them. Do you know where I can find a family tree of these gods?
If what you want is a family tree "per se", you can find a good dozen of them for free on the internet via your old pal Google. Here is a very solid one: 
Notice this tree (as a lot you will find) is a Mesopotamian one, and especially Babylonian. The presence of Tiamat and Marduk clearly signs the Babylonian nature. The fact Inanna is the daughter of Nanna and not Enki is another clear proof. A pure Sumerian tree would begin with An (the Anu of the Babylonian).
If you need some sources.
First imagine that anything centered on old Mesopotamia is vastly partially known, because it is OLD, terribly OLD, and what is written, was written on tablets most of them quite broken, or not so readable. And a lot of what we know of the Sumerian people is coming from the Babylonian or even later Hittites... if not in the Holy Bible!
As an example, one of the most notable pieces known and renown from Mesopotamia is The Epic of Gilgamesh which in its most well acknowledge format is Babylonian. The main source tablets we have are from the Assyrian city of Nineveh dating from king Assurbanipal. That demonstrates that most of out tablets source are either coming from unknown place or known and was a copy of older text (The Assyrians coming after the Babylonians). On a side note, the Sumerian language was used as latin and greek in paste time, not the linga franca but much more the intellectual language. That also makes difficult to know what is and what is not from Sumer.
The definitive Sumerian source is Samuel Noah Kramer, the best myth introduction is that book (freely available on sacred text):
Sumerian Mythology:
Study of Spiritual and Literary Achievement in the Third Millennium B.C.
You can also check:
The Sumerians, their history, cultures and character
There is there a lot of fairly solid translations of major mythes/stories, as well as a great introduction to the Sumerian culture. this is by far a better book than the former but it is not free. And quite dated (As Sumerian mythology is).
the Litterature of Ancient Sumer by Jeremy Black, most texts are in the ETCL
The World’s Oldest Literature A fairly good (but ridiculously over priced) book
Epic of the Sumerian kings: the matter of Aratta by Herman Vanstiphout, a solid tranlation of one of the 2 big cycle of stories, the other one being for Gilgamesh
Ancient mesopotamia by Leo Oppenheim, a fairly solid book on Mesopotamia in general
To avoid:
Inanna queen of heaven and earth by Diane Wolkstein and (unfortunately) Samuel Kramer a modern soup of transreading with no connection at all with Inanna.
http://etcsl.orinst.ox.ac.uk/ TONS of good material, with decent translations, lots of texts, poetries, songs. REALLY a great source. Really less expensive that Hallo's book...
