The part you are talking about happens in Chapter 24 -- the Missionary Circle is meeting at the Finch home.
I would say that the ladies of the Missionary Circle are not really all that sincere about caring for the Mrunas. My answer is based on their attitude towards the black people who live right there in Maycomb.
If the ladies sincerely cared about the Mrunas as fellow human beings, you would think they would care about black people in their own community as well. But instead, look in that chapter about the way they talk about the black people in Maycomb. Look at how Mrs. Merriwether talks about Sophy, for example.
No way! All they care about is going through the motions of their faith. They do what they do in order to think they are doing something good for someone else. However, all they are doing is living in a web of hypocrisy.
It is great that they get together to collect money and give it to their specific missionary so HE can go be good to the Mrunas. But if you listen to them talk about their own black or African servants who they have the opportunity to "minister" to on a regular basis, they have nothing but judgment, contempt, and degradation for them.
I believe Scout's narration demonstrates that she doesn't understand their side-talk, but she hears it. The comments these ladies make about their "darky" servants shows how insensitive they are to Africans who have needs.
In the book "To Kill a Mockingbird" the missionary ladies are a social club. They are not really political activists. They may have some good intention when it comes to the idea of helping the Murnas but they don't even have a full understanding of the situation.
For them the idea of helping poor Africans does not seem to even hold any relationship to the black people in America. It is easier for them to experience sympathy to a tribe far away then to relate t them in any way as if they were black people. They are much more of a group of social do-gooders than a group really intent on taking a political or supportive stand for any black people.
The ladies "believe" they are being sincere. However, they see all blacks as heathens who need converting to Christianity. Just as they see Rev. J. Grimes Everett as fighting a losing battle in trying to convert the Mrunas, they also believe that the Maycomb blacks will never be as good as the Maycomb whites. To them, Blacks are a race apart in thought, color, and morality.
This is Harper Lee at her satirical best. The old maxim, "Charity begins at home" is completely lost on these "Christian" women.