Re: mujhe choti wali pasand hai!!
Just to let you in on a little secret of arranged introductions. The “other” sister gets to be carefree because there is no pressure or expectations on her to “perform” for the prospective rishta. The girl who’s being “viewed” already feels self-conscious and on display. Being stern may actually be her being reserved. As for being more polished, don’t know what to say about that except, it might be a matter of perception.
But all of that is beside the point. The issue is why can’t you marry the younger one, when you went to see the older sister. Technically, there’s no reason you can’t other than it’s a bit insensitive to tell a family that you found one of their daughters to be “lacking.”
If you can take some time and re-approach the family a bit later by suggesting that you’re more compatible with the younger sister, why not pursue it. As for getting married before the older sister, it’s a social taboo that has no basis except for society then perceiving the older sister as lacking in some way and not being marriageable - are you willing to take on the world and say she’s a great person, except you liked the younger sister better?