The inheritance is the same context and condition. The inheritance has a set part that goes to the main children but also has a part were you can specify anyone to receive a certain portion not to exceed 1/3( more or less). We ran into this same issue when my father and cousin died. The person who got the one third actually got more than any of us as children due to the fact we had to split the amounts. My cousin did not have children so his adopted son received the 1/3 portion that could be specified to anyone.

The main issue with the inheritance was not with the child inheriting from the adopting parents but prohibiting the using of the child's inheritance from their biological parents by the adoptive parents to supplement their lifestyle.

Again the rules are there to help not harm. I can eat pork if it is the only means of eating for me to survive or sustain life. Islam is easy and natural we as human beings try to make it difficult. The laws are not outdated it is just the application has to be done with correct knowledge of the rule and the context then and now.

Act your age not your shoe size