Assalam o alikum wa rahmatullahi wa barakatoh.
I would like to ask: touching stepdaughter with lust and desire, is the marriage of mother and stepfather valid?
Can I have a direct evidence from Quran and Hadith plz.
As I have read difference of opinions.
Please help me.

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad, is His slave and Messenger.

Touching one's stepdaughter with lust and (sexual) desire does not nullify his marriage to her mother.

The opinion of the Maaliki and Shaafi‘i scholars– which many competent researching scholars considered preponderant, and we believe it to be preponderant as well in Islamweb – is that this act does not entail deeming the daughter’s mother unlawful to the husband, and thus, does not nullify the marraige. The Shaafi‘i scholar Ibn Hajar Al-Haytami said in Tuhfat Al-Muhtaaj: “…Mubaasharah (lustful touching that does not involve actual intercourse) such as a lustful Mufaakhathah (intercrural intercourse) is not the same as penetrative intercourse, according to the preponderant view, because it does not necessitate a waiting period. Accordingly, it does not necessitate the prohibition of marriage.” [End of Quote]

The Maaliki scholar Ad-Dardeer said in Ash-Sharh Al-Kabeer: “It is permissible for a man, who commits unlawful sexual intercourse with a woman, to marry any of her female descendants (Furoo’) or ascendants (Usool), and it is permissible for his father and son to marry her.” [End of Quote] If this is the ruling in case of unlawful sexual intercourse – we seek refuge with Allah from it – there is no doubt that lesser acts (of intimacy) like touching and the like are less serious (in this regard) with greater reason.

Allah knows best.