Is it permissible to conclude the marriage contract and marry the fiancée after having practiced masturbation and imagined intercourse with her and with her sisters or her mother? And is this considered zina (fornication/adultery) that would prohibit her [for him] according to the Hanafis and some scholars?

The aforementioned girl is not forbidden to the questioner because of what was mentioned, and it is permissible to عقد (marriage contract) with her, and she will become his wife, from whom he is permitted what a husband is permitted from his wife.

Imagining having intercourse with her mother or sister is not actual zina (illicit sexual intercourse), and it has no bearing on the prohibition of affinity by marriage.

The whispers of prohibiting this girl must be ignored.

Before the marriage contract, she remains a stranger to him, and it is not permissible to converse with her except out of necessity and within legitimate Islamic guidelines. Exchanging flirtatious and loving phrases is forbidden.

It is not permissible to keep her picture because that is a cause of fitna (temptation/strife).