Because it takes about 12 to 13 years for siblings' immigration to US, normally the sibling and his/her family members should apply for the immigtrant visa in the home country (e.g., China). So, you should indicate it in Items 62.a., 62.b and 62.c of Part 4 in I-130.
It seems you made a mistake in the I-130 that you did not fill the Items 62.a., 62.b and 62.c of Part 4. Thus, USCIS will not forward your approved I-130 to NVC. Thus, you need to file I-824 to ask USCIS to forward your approved I-130 to NVC for processing for your siblings to apply for the immigrant visa in China. This costs your extra money and time.
You need to file two I-824 for two I-130, one I-824 for your brother's I-130 and another I-824 for your sister's I-130. If you completed Items 62.a., 62.b and 62.c of Part 4 in I-130, USCIS would have forwarded your approved I-130 to NVC automatically and thus, you should have not needed to file I-824.
Of course, if your siblings are living in US with a legal non-immigrant status, they can file I-485 within US when the priority date is current (which takes about 9 to 10 years from now), and thus you don't need to file I-824. But it is extremely difficult for them to stay in US legally for about 10 years.
