its like that.. cos afterall, it is not your own house and you r out of your comfort zone. i jst started living with my mother in law too,but its my own flat, so its still ok for me. for me, i just do my own things, and she does her own. when i see her, i will just greet her and do what i shld do, like ask her if she ate already, or if she wanna eat etc.but my mother in law isnt like yours , never smile. she is quite friendly, but still no matter what,she isnt my own mum, so sure to have some awkward feeling arnd.
imp, i think what you shld do is just do what you need to do. greet her and try to talk to her . if she dont wanna talk then just do your own things. as long as you do what you need to do, no one can say anything.. some ppl is like that, they jst pull a black face and dont talk.