firstly, my comment was for everyone proclaiming what is and isn't inappropriate... it wasn't specifically directed at anyone.
deja: to be fair, you did say, "My mom and dad have never touched my thigh, that's just beyond weird..." and i'm here to say my mom touched me in ways that were normal in my house and affectionate in ways that you clearly consider inappropriate or weird.
yes; i'm a girl and yeah, it's probably more common in a house of other girls to bare your bits. but i also know of families that feel like: family is family and act the same around and are equally affectionate with other family members despite gender differences.
and, i was best friends in high school with a girl from such a family. her mom would even go so far as to discuss her brother's member when he brought girlfriends over. that's just how they were; there was no prejudice. (ftr, as i'd eventually dated him, the son/brother never felt uncomfortable with his mom's affection.)
as far as the vag flashing incident - maybe she's always walked around pantiless and he didn't notice it before now. i don't know; i wasn't there. but it's perfectly within reason for the OP to say, "for pete's sake mom, put on some damn underpants!" i'm not trying to dismiss the OP's concern, but again maybe because he’s developing he's just more cognizant of how some ways they've acted previously could be taken in a sexual context.
again, i think the easiest way to uncover if this is the OP's imagination or something more sinister, is to have the OP talk to his mom and sister and express his discomfort with their behavior. they (his mom and sister) may not realize what they're doing makes the OP uncomfortable.