[Avodah] Rav Moshe Shternbuch: Do Adopted Children Sit Shiva For Their Step Parents?

[Prof. Levine]

 From http://revach.net/article.php?id=1529

Although they are technically not related or considered family, Rav 
Moshe Shternbuch says (3:374) that if an adopted child grew up with 
his step parents and considered them to be his parents he should sit 
Shiva and keep all halachos of Aveilus upon their death.  While in 
Aveilus there is a concept of showing your sadness by keeping some 
rules of aveilus, Rav Shternbuch says that in this case one should 
keep all the details of aveilus exactly like an Aveil.

He learns this from Rabban Gamliel in the Mishna who sat Shiva for 
his slave, Tavi.  The Rashba says that Rabban Gamliel sat shiva 
because he felt towards Tavi like a father would towards his 
son.  Rav Shternbuch reasons that if feelings of closeness caused 
Rabban Gamliel to sit shiva for an Eved who has no Din of Aveilus 
than certainly for the people who raised you and do have a Din of Aveilus.

He adds that this is a Mitzva not a Chiyuv.  Therefore you should not 
be Meikil when it comes to the Din of Aninus of not putting on 
Tefilin or excusing you before the Kevurah of any other 
mitzva.  Lastly he says if you don't feel like a son to your step 
parents then you should just keep some halachos of Aveilus to share 
the pain but do not keep all the halachos like and aveil would.