Karen Williams and Patty King said the musician was given "foreign substances to induce his premature death" by his business manager Laverne Toney.
Lawyers for King's estate said the claims are unfounded and disrespectful.
Nevada's Clark County Coroner said it would investigate with the Las Vegas Police Department's homicide division.
"At this point, we don't have evidence that these allegations of foul play will be substantiated," Clark County Coroner John Fudenberg said.
"However, we are taking them very seriously and will be conducting a thorough investigation.
"We are co-ordinating our investigative efforts with the homicide division of the Las Vegas Metropolitan Police Department. I expect the investigation will take a minimum of six to eight weeks."
However a statement from the Las Vegas Police Department said: "Until such time as the Clark County Coroner determines Mr King's death to be from other than natural causes, the Las Vegas Metropolitan Police Department is not moving forward with any investigation."
King died in his sleep at his Las Vegas home on 14 May, aged 89.
At the time, his doctor and the coroner said he had died of a series of small strokes connected to his Type 2 diabetes.
But the guitarist's daughters alleged that King's personal assistant Myron Johnson and Toney gave him medication to induce diabetic shock.
The women added that "King was sequestered from all family members" in the week before his death, and that Toney and Johnson were the only people with him.
Toney, who is the executor of King's estate, shrugged off the daughters' claims.
"They've been making allegations all along. What's new?" she told the Associated Press.
A lawyer for King's estate also dismissed the accusations as "ridiculous".
"I hope they have a factual basis that they can demonstrate for their defamatory and libellous allegations," Brent Bryson said in a statement.
Earlier this year, Patty King, Karen Williams and a third daughter, Rita Washington, went to court accusing Toney of neglect, but the case was dismissed because of a lack of evidence.
King was born on 16 September 1925 to sharecroppers and worked in the cotton fields as a child before picking up the guitar.
Considered one of the world's greatest players, he was known for his sharp single notes and vibrato on the electric guitar he christened Lucille.
Known for songs such as The Thrill Is Gone, Three O'Clock Blues and Darlin' You Know I Love You, he influenced generations of guitar players, and was inducted to both the Blues Foundation Hall of Fame and the Rock and Roll Hall of Fame.
He was married twice and had 15 natural and adopted children, 11 of whom are still alive.
His funeral is scheduled to be held on Friday in Mississippi.
The Clark County coroner said his investigation would not delay the service.

Jersey made it past the opening round for the first time in their history after coming back from a goal down to beat Salisbury 2-1.
Josie Steinson put Salisbury ahead before half-time, but two goals from Fiona Anderson, the second in the final minutes, sealed the win.
Meanwhile, Guernsey won 4-1 away at Warminster-based West Wilts.

Gwynedd council's cabinet discussed a £9.27m overhaul of schools in Bala over falling pupil numbers at a meeting on Thursday.
Ysgol Beuno Sant, Ysgol Bro Tegid and Ysgol y Berwyn could close and make way for a school for three to 19-year-olds on the site of Ysgol y Berwyn.
There is also a proposal to amalgamate control of Ysgol Bro Tryweryn, Ysgol Ffridd y Llyn and Ysgol OM Edwards.
The cabinet approved plans to open consultations ahead of a final decision in September.
The new campus could open in September 2018.
US authorities are investigating the death of blues legend BB King after two of his daughters claimed he was poisoned.
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Jersey and Guernsey's women's teams are both through to the second round of the EH Trophy after first-round wins.
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Plans to close three schools in Gwynedd will go out to consultation.