You don’t specify if this is at the pool or beach, or in other circumstances.
There is nothing weird with wearing a thong bikini at the pool or beach, even with family. People all over the world do it (especially in Europe and South America). It is startng to be popular in the US too.
If she wears it at home, it’s s bit unusual, but I would not go as far as calling it weird. Families have different values and some don’t find this offensive. I grew up in a very laid-back family. We only had one bathroom in the house. Sharing it was expected, since we all had places to go in the morning. It gives you a different perspective, and you realize that seeing a nude relative or being seen nude by them is no big deal.