There's a lot of reasons and your sister would be aware of the strategic ones.
No structured/team sports makes a massive difference, especially at weekends
No alcohol/pubs/clubs/big gatherings like festivals, parties etc - alcohol related accidents and violence are a huge part of overnight work normally
V few extreme sports and activities going on, less climbing, mountaineering, water sports, horse riding, trail cycling etc etc
Many walk in centres and MIUs have increased their hours and scope to see more minor ailments away from main hospitals, some even increasing their usually limited xray facilities to prevent having to refer limb injuries on.
Most hospitals have set up different admission arrangements for patients so that they go straight to a ward rather than through A&E if they have been referred in by the GP.
Community and mental health services have all been beefed up to keep patients away from main hospitals, palliative care teams have been increased via redeployment too
There is real concern that many people will be being stoic when they really shouldn't be, and that many may be very unwell and not seeking the help they need. Lots of people are genuinely frightened to leave the house in case of catching c19, but they may die of their untreated health problems sooner because of that.
Many many 'uneccessary' A&E visits in non pandemic circumstances are because of poor health/social care provisions in general - people unable to get a gp appointment, unable to get referred to mental health services or CAMHS, unable to access NHS dentistry, unable to see a health visitor or school nurse, unable to get food/drink/blankets and living rough, unable to get parenting support, unable to get palliative care support. The list goes on. Many people come to a&e because it's open and someone will listen. It's convenient to blame the vulnerable and explain the misuse of emergency care as someone's fault but it's often a lot more complicated than that.
Rather than being pleased that none of those people are coming in now, we should all be worried about where those people are, how they are and what on earth we're going to discover when the world gears back up again.