There are two related questions on this topic:
- Was marrying brother's wife an accepted in the society of the Ramayana or pre-Ramayana era?
- Is levirate marriage allowed in Hinduism?
First one is mainly focused on Ramayana and pre-Ramayana timings. Second question was marked as duplicate of first one. Second question is asking in general and not related to a specific timing.
This question is primarily focused on Dwapara Yuga and Kali Yuga.
Consider the following statement by Bhishma during a conversation with Yudhishthira
A woman in the absence of her husband, takes his younger brother for her lord;
From the statement, it seems clear to me that accepting husband's brother as lord in the absence of husband is allowed for women during Dwapara yuga.
But, is there any other scriptural statement that either support or oppose such acceptance pertaining to Dwapara yuga?
What do scriptures say about it in Kali yuga?
Note: This is not at all related to the Niyoga dharma.