WA Tenant subletting issue

[Bob Malecki]

I have a tenant in my Bremerton, WA duplex who apparently is subletting her apartment to her sister and sister's family without any notice to me. My lease does not allow this without proper notice. My tenant is paying rent on time, but tenant in the other half says that the sister and her husband are "acting funny" and leaving their 3 kids alone for extended periods of time, perhaps a drug useage issue. 

Any thoughts on my best course of action on this? I really don't want to have to evict them, but my tenant in the other unit is getting concerned/scared. 

Thanks

Bob