Some victims are being groomed to copy adult pornography, say the Internet Watch Foundation (IWF), with children as young as three being targeted.
Campaigners are now calling for livestreaming services online to better protect children from abuse.
How big is the problem?
Around one in 30 instances of "self-generated content" found online between September and December 2020 involved siblings, say the IWF, which found 511 examples in that period.
The IWF say coronavirus has accelerated the volume of online abuse, with a greater demand for abusive content online colliding with a greater amount of time spent online by children.
The pandemic has also seen a rise in the use of livestreaming services by all ages.
How are children targeted?
Often, criminals and paedophiles begin grooming children on social media and gaming platforms, before encouraging them to try video chat or live streaming sites, where abuse typically escalates.
In cases of sibling abuse, the older child is typically coerced into abusing their younger brother or sister.
The IWF say a common misconception is that abuse involving siblings is a problem limited to poorer countries.
The majority of examples found by the IWF were in western countries including the UK and the US.
What protections are in place?
The government has taken steps to strengthen protections online, including putting forward an Online Harms Bill which compels companies to enact proper safeguarding measures.
Under the new bill, watchdog Ofcom will have the power to:
- Fine technology companies
- Block access to online sites and services which don’t protect users
NSPCC online-safety-policy head Andy Burrows told BBC News the new bill could be a "world-leading piece of legislation".
He added, however, that it is imperative the bill follows through with its promise to give Ofcom the power to step in and take action.
The NSPCC have said that the race to bring livestreaming and video-chat services to the market quickly over the past year has led to a prioritisation of profit over making platforms safe for users.
What are platforms doing?
Some experts have warned that online platforms are not doing enough to combat abuse of children online.
Speaking to BBC News, Mr Burrows expressed particular alarm over plans to introduce end-to-end encryption on Facebook services.
End-to-end encryption is a secure way of sending text, images and other forms of media so that only the sender and receiver can see them.
Facebook’s plan to implement this on new video-chat service Rooms in May would "significantly compromise the ability to detect child abuse", he told the BBC.
What should I do if I suspect abuse?
If you suspect your child might be the victim of grooming online, you can get in touch with the NSPCC at 0808 800 5000, email [email protected] or fill in their online form.
The NSPCC says parents should look out for the following signs of grooming in their child:
- Being very secretive about how they're spending their time, including when online
- Having an older boyfriend or girlfriend
- Having money or new things like clothes and mobile phones that they can't or won't explain
- Underage drinking or drug taking
- Spending more or less time online or on their devices
- Being upset, withdrawn or distressed
- Sexualised behaviour, language or an understanding of sex that's not appropriate for their age
- Spending more time away from home or going missing for periods of time.