I don't know about the costs of DNA testing, but there are many sources available out there, these days.
As far as interacting with the sister, if you aren't concerned about the uncle's Will (good for you!), then there is literally no reason (legal, or otherwise) to communicate with her. Seriously. And, if you choose this route, it means not answering her calls, responding to emails, blocking her texts, etc. If she's toxic to you, there are the options of: a) exposing yourself to manipulations or other toxic behaviors by allowing communications, ...
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