I (M35) saw my sister (F33) topless on the beach

My sister came on holiday with my girlfriend and I last summer. The first couple of days at the beach we're normal, the girls wore bikinis, some smaller than others but nothing out of the ordinary. On the third day, my gf said to me that she wanted to go topless because she didnt want to get tan lines and my sister said she wanted to do the same. I assumed they meant lying on their front which is how it started. After a while they both rolled over and sat up, I couldnt believe what I was seeing. They asked if I minded and I said no so they carried on. Basically spent the rest of the holiday topless whenever we were at the beach.