The Ha Ha's

"The Ha Ha's" by Michael H. Hanson
A weird tale about about four siblings' supernatural childhood guardians haunts the youngest of the kids, an elderly woman now in her nineties, who lives alone and regularly visits her dying sister, the only other survivor amongst the two sisters and two brothers.
The youngest of the four kids, she had never actually seen The Ha Ha's that her older siblings whispered about behind closed doors. She grew up thinking these poorly described creatures were a childhood whimsy, no more real than Santa Claus or The Easter Rabbit.
Her dying sister, suffering from dementia, finally confides the truth to her, a bizarre confession of flashbacks, assaults, combat survival stories, deep antediluvian caves and macabre mystical native-American petroglyphs and eldritch palaeoart, and much more.
Attacked by a ruthless street gang on her walk back to her apartment, she is forced to put this new arcane knowledge to the test by desperately attempting to summon The Ha Ha's, creating a tenebrous bridge joining reality with a horrible, accursed Hell realm.