Yocheved's relationship to Amram
Regarding Yocheved's relationship to Amram, Exodus (6:20) certainly implies that she was his aunt:
וַיִּקַּח עַמְרָם אֶת-יוֹכֶבֶד דֹּדָתוֹ, לוֹ לְאִשָּׁה
And Amram took Jochebed his father's sister for a wife.
Regarding your question:
I most definitely think this cannot be the case. Because the Torah She-baal peh` states that Amram was one of the four people who never sinned in their life
It should first be noted that Aggadic Midrashim are not necessarily based on ancient tradition.
However, that is besides the point, since maintaining that he married his aunt does not necessitate rejecting the Midrash. Regarding the possible violation of Noahide incest laws, some state that she was a paternal aunt, rather than a maternal aunt. (Yalkut Shim'oni B'reshit: 25, cf. Rashi Exod. 6:20).
Furthermore, it is not clear that a gentile is forbidden from marrying any aunt. R. Akiva (arguing with R. Eliezer) cited in Sanhedrin (58a) is of the opinion that a gentile is permitted to marry his aunt! (Incidentally, this is the ruling of Rambam Hilkhot Issurei Biah 14:10).
Thus there the issue of marriage to his aunt does not seem to be evidence whatsoever that Yocheved was not Amram's aunt.
However, in spite of all of that the answer to the initial question "who is Yocheved's father" is actually not obvious.
The Torah seems to say that she is the the daughter of Levi (Numbers 26:29):
וְשֵׁם אֵשֶׁת עַמְרָם, יוֹכֶבֶד בַּת-לֵוִי, אֲשֶׁר יָלְדָה אֹתָהּ לְלֵוִי, בְּמִצְרָיִם
And the name of Amram's wife was Jochebed, the daughter of Levi, who was born to Levi in Egypt.
Indeed, this simple understanding is affirmed by numerous commentators such as Ibn Ezra in his extended commentary to Exodus (2:1), and Rabbenu Bahya is his commentary to Exodus (2:1).
However, for reasons wholly unrelated to those in the questions, R. Shadal, writes in his commentary to Exodus (6:20) that Scripture did not list every generation in Levi's genealogy. Instead, some of the names of his descendants became family names. In this vein, he cites an opinion that until Levi died, the family of his descendants was knows as "Levi", after which the family split into three; Gershom, Kehat, and Merari.
According to this original explanation (which It appears Ibn Ezra and Rabbenu Bahya were specifically trying to preempt), Yocheved would not necessarily have been a daughter of Levi, the man, but any descendant of the family born until his death.