The rule is that you must answer every question the committee asks you, otherwise you can be held in contempt of Congress. However, the committee cannot demand that you answer a question if, in answering, you will incriminate yourself.
You still have your Fifth Amendment right when you testify before Congress.
Liz Ceney is not currently under investigation.
This assumption is based on disinformation being pushed by Trump to get the FBI to investigate Cheney based on an ethics complaint filed by GOP members.
https://thehill.com/homenews/administration/5055401-trump-accuses-liz-cheney/
It is stated in the Ethics Complaint" The allegation is that Cheney violated the “anti-contact rule” of legal ethics, by communicating with Cassidy Hutchinson without going through her lawyer. And, because of that supposed violation, the Subcommittee urges the FBI to investigate Cheney for the crime of witness tampering (p. 117).
That accusation is based on a selective quotation of the anti-contact rule, leaving out the very words that prove that it does not apply. The effect is to create a public show that may well serve political purposes, but should be a complete nonstarter under any bona fide legal review.
https://www.justsecurity.org/106004/ethics-complaint-cheney-hutchinson/
The problem is, though, that she didn’t, because the rule applies only to lawyers who are representing clients – which Cheney was not.
Both the House report and the America First Legal complaint conspicuously lop off the first seven words of the sentence they quote from Rule 4.2(a), which make it clear why the rule doesn’t apply:
During the course of representing a client, a lawyer shall not communicate or cause another to communicate about the subject of the representation with a person known to be represented by another lawyer in the matter, unless the lawyer has the prior consent of the lawyer representing such other person …. (emphasis added)
Cheney was not representing a client.