Legal procedure to file for divorce & child's custody

you have to file a divorce petition in ground of cruelty mutual consent or adultery
accord to a judgement of hon'ble supreme courts says if a child is under 7 year the custody of the baby is under her mother she is good care taker

I will need more information to be able to respond to your query specifically. is her husband willing to divorce her too? and is he willing to give the custody of their child to her? Moreover, i will need to know if she is looking for maintenance, which is her prerogative.

Well,
If it is contested divorce then she need to sent a legal notice to her husband through an advocate... Then file a divorce petition in family court stating the grounds for the divorce.... The child custody will come to mother becoz child is below five years and he need spl care of mother rest you can meet an advocate and discuss the matter

The divorce petition needs to be filed before the family court under whose jurisdiction your sister resides. Along with the petition you need to file an application for child custody. The petition must contain the specific grounds for divorce specifically like cruelty, harassment, or anything else. If your sister is living separate from her husband the. The petition shall also mention this and the reasons for residing separately.
The application for child custody shall inform the court that why leaving her child with the father is not good for the child.

First your sister have to approach a matrimonial expert lawyer with all relevant documents such as marriage certificate ,, photographs ,id proofs , then a petition will be filled in the court by the lawyer .