He's Told To Marry Dead Wife’s Younger Sister

Have heard or read about weird things happening in our society, most of which are attributed to culture, tradition or religion.
Really, this one baffles me and my friend is beginning to get worried as to what to do.
Barely six months after the demise of his wife due to complications from childbirth, the in-laws are asking him to take his dead wife younger sister in as new wife.
Their excuse is that the sister will take care of new born better than an entirely new woman.
In addition the lady in question has accepted to marry her dead sister husband.
my friend is rather shock to make a decision.