Edit: This has just officially changed (sorta), see here. Ivanka now has an official White House office, but is not officially a government employee. Honestly I have no idea what exactly that means as it's in a super grey area.
I will be leaving my old answer below as it was the correct answer at the time the OP asked the question.
Officially Ivanka Trump has no White House role, not even as an advisor. If you look at the wiki article's wording, it's "she is considered to be an adviser", not she is an advisor.
However, in reality she obviously has the close ear of the President and does advise him on issues. From this standpoint it's not uncommon for a president (or anyone really) to have people give them advice from outside their team. For example, presidents will often times speak with previous presidents, even though previous presidents are not part of the official team.
In the same way, it is not uncommon for Presidential family members to be unofficial advisors to the president. Its unavoidable. As a simple example, the first lady has no official position in the office of the president (she has her own office that is not defined by the constitution), but the first lady still will obviously discuss policy with the president.
Ivanka though does have attributes in her role that do make it unusual. As mentioned in those articles Ivanka is generally much more hands on than your average unofficial advisor. For example, the first lady generally does not sit in on policy meetings with foreign heads of state.
For the ethical side of it... it's hard to say. As mentioned, she is not officially part of the White House staff, so the restrictions on her are much looser, but there are clearly some actions that raise eyebrows. The most concrete thing here is the federal anti-nepotism statutes that restrict employment of relatives. This actually came up when Ivanka's husband Jared Kushner was officially appointed as a senior advisor. Again however, since Ivanka is not officially employed by the White House it's hard to bring a solid case.