Donald Trump's older sister, federal appeals court judge Maryanne Trump Barry, received a threatening letter in Philadelphia, sources told NBC News on Saturday.
The letter arrived on Friday, according to the network, one day after Trump's son, Eric Trump, received a suspicious piece of mail containing a white powder.
There was no white powder sent to Trump Barry, although the threat included in her letter was similar to that given to Eric, a source familiar with the investigation told NBC. Eric Trump's letter demanded that his real estate mogul father drop out of the Republican race for the presidential nomination.
It remains unclear if both letters have the same sender. The FBI and Secret Service are investigating the incident.
The mail sent to Eric Trump was postmarked in Massachusetts, police sources said, but preliminary tests deemed the substance non-hazardous.
As he moves towards securing the Republican nomination, Donald Trump has also caused a backlash with his at times controversial comments. Opposition to his campaign has even been seen by senior GOP leaders, who reportedly are considering an independent run to try and thwart him.
On Friday, former Republican presidential candidate Mitt Romney announced he would vote for Trump's rival, Ted Cruz, in an attempt to block Trump from becoming the presidential nominee.