I am COVID cautious. I wear my N95 100% of the time I'm indoors (except at home). I only eat in outdoor restaurants and cafes. We test whenever anyone in the household is ill. My children don't get taken to crowded ball pit type places but we do visit indoor things with masks on at non peak times.
COVID is a neuro invasive, vascular disease, and it's clear that every infection, however mild, leaves a mark on the body. It also damages the immune system and can cause cognitive decline. Just because the initial symptoms are mild doesn't mean you have been undamaged. Mainstream media has chosen to avoid the topic but there are articles highlighting the research/evidence on cognitive damage, risk of heart attack and strokes, immune system damage, long COVID etc if you look. And over 400,000 peer reviewed papers.
It makes sense to avoid catching it repeatedly and wearing a mask indoors really is not a problem. I will continue until a more effective vaccine is developed.
What breaks my heart however is children being sent to school with zero airborne mitigations and school admins that prioritise attendance over health. As a result, schools are infection pits, and children pass it on to their wider family and so it spreads. Yet since every infection is damaging, we should be doing everything we can to minimize multiple infections in our kids. In schools that means ventilation at least, preferably combined with air purifiers, and keep sick children home. COVIDsafetyforschools.org is a great resource - please have a look and ask your children's schools what measures they are taking.
The data is very clear (eg long term sick leave stats, not just in UK, and NHS staff illness absence data) that across the population, illness absence has risen continuously since 2020, and it's not because of vaccines.
You should instead be supportive of your SIL, mask up around her and show understanding. She's certainly not wrong to be cautious. And perhaps consider if it's wise to be blase about catching such novel virus about which we have much more to learn.