This is wrong on so many levels.
That child is in a very difficult situation. No doubt she was 'pressured' by her parents to get baptised, and at such a young age did not have any real understanding of the seriousness, demands and the consequences for making such a decision. Now she is being pressured by her parents and others to shun her sister. Deep down she is most probably distressed about it and has no choice but to put on a brave face. It must be terrible for the older sister too.
And as for the parents, sheeeesh, can't they see that their family is being torn apart!
I'm just glad that I didn't have to go through all that crap during my pre-teens.