The teenager wed the jihadist over the messaging service in September 2016, the court heard.
She had already been arrested on a terrorism charge in August after authorities thwarted her plan to join the ISIS member in Syria, but was released on bail.
Between December and April, the duo allegedly conspired to carry out a terror attack in Britain and the girl planned to receive hand grenades and a firearm.
She is accused of receiving instructions on how to use the weapons as well as asking another person for help in carrying out the planned attack.
The teenager, who cannot be named for legal reasons, was taken into custody on April 12 when she was formally charged with the earlier terrorism offence.
The charges over the UK attack plot followed.
The girl is not the first young Londoner to allegedly be involved with ISIS.
In 2015 three teenager girls left their homes in east London and are believed to have travelled to Syria to join the jihadist group, following a classmate who left a few months earlier.
Two of the trio were in touch with their families later that year to say they had married IS fighters, the Guardian reported at the time.
Around 850 people with links to the UK have travelled to Syria to take part in the war, according to government figures.