information regarding s1 and s2 visa for my kids and sister in law.

hello, i have 2 kids, my son is 1 mnth old and daughter is 3 and half year old. i wants my sister in law to go with us and help me because i am also student in china. so my husband will send documents for them. my sister in law can get s1 of s2 because we want she stay with us for long time about 2 years.so how i can get long term one for my husband,s sister or she can only get s2. kindly guide me.