What is the ruling on marriage according to the school of Abu Hanifa, which prohibits marriage due to affinity (muṣāhara) through lustful looking at the mother-in-law or her sister before marriage, if the young man engaged in that with the mother of his wife while she is deceased?

It is not obligatory to follow the Hanafi school of thought regarding the invalidity of a man's marriage to a woman if he has committed adultery with her mother or daughter, or touched them with desire, or looked at their private parts. It is preferable not to mention a specific madhhab in the marriage contract.