Is it normal for my sister to not wear pants?

You should probably just say it to her nicely. Personally I dont do that im a girl aswell, I normally do take of pants, jeans or leggins aswell and put on p.j's pants if its cold or p.j's shorts if its warm if i wanna relax maybe she cud do da instead?

Hmm I can understand going round the house in underwear and a t-shirt...that's I've been in all Easter holiday :P. But doing the same at other peoples houses? If the host doesn't then I guess...but nah that's not normal.

I do it all the time and my brother hates it. :-P I just do it to be comfortable and because pants are restricting. I say, just ask her nicely if you want her to stop.

Not really. In your own home, it's acceptable, but if she's in a family home or a friends, she still needs to be respectful of the others in the house.

well... i do the same the moment i go home or at my gf but... not with random friends. Not that i wouldn't want it or i would mind, just cause i think they may mind, well the female friends anyway :)

This is in very normal in my opinion. Me, a girl myself, has a habit of being pantless at home. I also remove my pants when I stay at my friends' place, but only at night when we are about to sleep.