Catherine asks, "Is this just rumor or fact that Wyatt left Urilla and the baby to die? It is said this is what caused Urilla's family to blame Wyatt for her death."

There absolutely is no historical basis for those claims.  Wyatt did not leave Urilla (or "Aurilla") and a child to die, as he remained in Lamar for several months following their passing.  The deaths, from typhus and/or childbirth, occurred in September or October of 1870 and Wyatt stayed in Lamar until March, 1871.

The idea that Urilla's family blamed Wyatt for her death is pure speculation, probably based on the fist fight that occurred among the Earp brothers, the Sutherlands and the Brummett boys months later (the Brummetts were friends of the Sutherland brothers).  No cause for the fight has been discovered.  There is evidence of "bad blood" between the Earps and Brummetts as early as August, 1870, when Nicholas and Wyatt were legal agents in a case against Granville Brummett and others for bootlegging, and it is uncertain on whose side the Sutherland boys were fighting.

(Many thanks to historian Gary Roberts for giving us much of this information.)