How do you plan to do it anonymously, considering what you now know?:
1. There are already multiple database containing both your parents, you, and a linkage between you and them indicating parentage. So, prior knowledge: Alice and Bob are parents of Charlie.
2. If Charlie's parents have taken a DNA test, there already exists a database linking their DNA to their name. So, prior knowledge: Alice's DNA belongs to Alice, Bob's DNA belongs to Bob.
3. If Charlie takes a DNA test totally anonymously and perfectly untraceably, it will still show up as, child of Alice and Bob's DNA. So, knowledge now includes: Charlie's (anonymous) DNA is the son of Alice and Bob's DNA
4. From these pieces of information, it is trivial to de-anonymize Charlie's DNA, linking it to Charlie's identity: the only person it could belong to is the son of Alice and Bob, and the son of Alice and Bob is already known from point 1.
I think in my case I'm just not that concerned by the hypothetical because my parents haven't done sequencing/genetic screening and also aren't likely to. I guess the main question is how far out in my family tree I have to think about that. (Also has implications for my descendants, I suppose...)
1. There are already multiple database containing both your parents, you, and a linkage between you and them indicating parentage. So, prior knowledge: Alice and Bob are parents of Charlie.
2. If Charlie's parents have taken a DNA test, there already exists a database linking their DNA to their name. So, prior knowledge: Alice's DNA belongs to Alice, Bob's DNA belongs to Bob.
3. If Charlie takes a DNA test totally anonymously and perfectly untraceably, it will still show up as, child of Alice and Bob's DNA. So, knowledge now includes: Charlie's (anonymous) DNA is the son of Alice and Bob's DNA
4. From these pieces of information, it is trivial to de-anonymize Charlie's DNA, linking it to Charlie's identity: the only person it could belong to is the son of Alice and Bob, and the son of Alice and Bob is already known from point 1.