What's your source? The Handbook of Texas Online's bio for Ima Hogg lists three brothers by name and birthdate, but no mention of any sisters.
http://www.tsha.utexas.edu/handbook/online/articles/view/HH/fho16.html
Another source debunking the myth that Ura Hogg existed:
http://www.ci.austin.tx.us/library/ff_texa10.htm
A Texas historian confirms (last sentence):
http://www.texasescapes.com/DEPARTMENTS/Guest_Columnists/East_Texas_all_things_historical/McDonald_Hogg_82700.htm