UK Tier 2 work visa - applying visa for my step daughter

[expat_indian]

Hello Experts,

Reaching out to this forum for information and advice.

Background:
I am an Indian citizen and working in UK for 2 years on Tier 2 work visa.
I am planning to marry, and my to-be wife has a daughter (currently aged 5) from her previous marriage.
The court orders at the time of her divorce (2019) states that my wife will have legal custody and sole responsibility of her daughter and her husband is allowed to visit the child twice a year.
The birth certificate of the child bears the name of biological father.
Since the birth of her daughter, my wife is taking care of all the needs (housing, food, medication, schooling, etc).

Current situation:
After my marriage next month, I am planning to apply for dependent visa for my my wife and my step daughter.

Going through information on various websites has confused me a bit and I have following questions in my mind for dependent child visa:
1. As my wife has legal custody and sole responsibility for her daughter (based on the court order for divorce), what other documents are needed at time of dependent visa filing?

2. Some threads list 'permission letter from biological father to take daughter to UK' a requirement. So, is it required?
What should be the format of this letter and is a notarised affidavit sufficient?

Any helps on this matter would be highly appreciated.

Regards,
Expat Indian