I have to admit, from what you've said it seems to me that you are very careful and no one should be able to see anything.
There is the chance that you are moving after you have sit down and exposing something, but that would have to be quite a big move.
I would suspect that it is simply some people having very active imaginations.
Here's an example.

My sister recently told me I had to cross my legs when I was wearing my kilt because she could see my underwear.
I informed her that crossing your legs often made everything more visible. And I called her out. I asked her what colour underwear I was wearing.
'Black' She replied.
'No. Bright red and yellow tartan.' I said. (The kilt was grey tweed, so there could be no mix-ups.)

Perhaps you could have a quiet word with her and say that you are very careful and perhaps even lie and say you were wearing underwear so they shouldn't have been able to see anything at all.
Inform her that you will accede to her wishes if you are in her home, but that if you plan on going anywhere in public together you will continue to wear whatever you want.


Whatever you decide to do, good luck.