Our Current Arrangement

I'm currently in a secret, i****tuous relationship. About two years ago, my older sister (currently 54) and I (52) became a couple.
We'd both been going through a rough patch. I had just finalized my divorce from my now exwife, who had been cheating on me for years. during the divorce process I dated here and there, but it was rarely satisfactory. My sister, meanwhile, was going through something similar. She had concluded her divorce from her second, abusive creep of a husband. She'd been left penniless from the ordeal, so when she asked if she could stay at my place for a while, so she could get back on her feet, I said sure. I had a spare room now, and we both have children but they are adults and mine live abroad. We could use the company, I thought.
We always got along, but we were never really that close. So it was smooth sailing from the get go, she had her room and I had mine. At some point we both brought dates home and it was all very casual.
We started to get attracted to each other when we started finding comfort for our insecurities and traumas in each other. We started to spend more time together, sporadically dropping compliments every now and then, getting snuggly over movies. It didn't feel wrong. It felt exciting.
The breaking point came the night after she landed herself a much needed job. We celebrated over wine and dinner. That night, a weird memory popped up. The one time that, as preteens, we experimented kissing and groping, something we had kept in secret and hadn't even spoken of in so long, I didn't even remember. However, the conversation heated up with that, and then... I made love to my sister that night. More than once. And it was the most satisfying sex any of us had had in ages.
The taboo around our relationship bothered us for a while, before we just figured "fuck it, be with the person who loves you and wants you". We've been through enough people that didn't, so if the person bound to love us were ourselves, we'd just go with it.
Since then we've felt more alive and fulfilled, kind of like a young couple on their honeymoon. Sex is open, explosive and fulfilling, we take care of each other and have the greatest time together. Then the pandemic hit at we were not in a position to visit our children or have them visit us, so we still haven't faced the challenge of hiding it from them (because we obviously won't tell them the truth. We've kept two rooms set up for appearances). Only a close friend knows about us. They were very shocked at first, but have since accepted us and kept it secret.