effort or efforts

That's an interesting one. In standard usage, either is quite normal. I have a personal preference for the singular form but only because if one transposes other similar terms in that sentence such as "help" or "aid", the plural form obviously doesn't work.

"Effort" as a non-countable noun (these are always singular) means energy or work: to lift a heavy stone requires much effort. To use "efforts" in the plural is to use the noun in its countable sense, that is, to refer to individual attempts to help convert your sister. Are you talking about individual efforts, or are you talking about one collective effort?

Did your sister get converted? "XYZ religion" sounds kind of scary. is it a cult?

Thanks, contrex.

I'm talking about individual attempts.
Yes, she was converted to Christianity. (I used XYZ religion because I'm afraid some of the members may accuse of trying to convert them.)

There might be a subtle diff; where it was done all at once or whether a number of separate tries were launched

Tang, forgive