The letters were intercepted, three at a time on two separate occasions, at a mail screening centre and the powder was found to be non hazardous, Harrow Crown Court was told.
One was addressed to Mr Clegg and on the envelope was written ''devil worshipping'', ''freemason'', ''sex with 30 plus women'' and ''your poor Catholic wife and children''.
The defendant, who is known as Sister Ruth Augustus, denies six counts of hoaxes involving noxious substances or things.
Mark Kimsey, prosecuting, said Augustus, 71, of Leyton, east London, accepts that she sent envelopes with letters in them but says police put the white powder in them.
He told the jury: ''The issue is whether she herself put the white powder within the envelopes, and, if so, was it with the intention to induce the recipient to fear they were hazardous.
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''Albeit this is a hoax, it's a serious matter, when what was sent was white powder, with the intention to make the person believe they were receiving a noxious substance.''
Mr Kimsey said three envelopes were intercepted at a mail screening centre in east London on June 17 2011.
A worker was checking mail at 5.30am when three envelopes raised concerns.
The second was to Baroness Scotland, and had a swastika on it, and two crosses, and ''stop this evil devil worshipping''.
The third was to Baroness Kennedy, and was endorsed with a swastika, and ''stop these evil devil worshipping freemasons''.
The envelopes contained a gritty substance, but it was found they had already tested negative for anthrax, and specialist police who were called in found them to be non hazardous.
On October 1, at the same place, three more envelopes were found, addressed to Mr Clegg, Baroness Kennedy and Edward Leigh MP.
The envelopes carried similar endorsements and slogans and contained white powder which was found to be non hazardous.
On December 7, Augustus was arrested at a hotel in north west London where she was staying and told police ''It's a load of lies'', Mr Kimsey said.