What is the plural form of sister-in-law and what is the rules of it's plural form?

The plural for for the compound noun sister-in-law is sisters-in-law.

The rule is that the 'sisters' become multiple, but the relationship 'in-law' does not become multiple, it is still the same relationship to a siblings spouse.

Note that you will hear people say, 'my in-laws'. This term is used when the 'in-laws' are relationships of varying combinations of sister, brother, father, mother-in-law and the abbreviated form is easier to say.