Why is this plural?
Along with many neuter nouns, "søsken" can be singular or plural. Here, it only makes sense to have it be plural.
I thought that was only monosyllabic neuter words? Is there a guideline or rule for words like this?
It's a rule of thumb, but like all rules of thumb, there are exceptions, like "søsken." You should memorize the rule word-by-word.