My sister's in-laws are my sister's children. It may be your sister's son's in-laws, or it may be your sister's daughter's in-laws.

If it is a close in-law, it should also be called in-law. Older than you, you should be called elder sister or sister-in-law If you are younger than you, you can call your brother, sister and brother-in-law. This situation is very common in rural areas. No matter my cousin's in-laws, they are all in-laws.