In the Name of Allah, the Inspirer of Truth. Assalamu Alaikum Warahmatullahi Wabarakatuh
Yes, it is permissible to marry the husband's brother after termination of Iddat'. It is extremely praiseworthy to raise one's step son as a father though he would of course not be regarded as the biological father.
And Allah knows Best Wa Alaykumussalaam Wa Rahmatullahi Wa Barakatuh