Looking at the source in the Torah (Bereshit 48:15-16) seems to indicate something else. It isn't that Yaacov Avinu is making the sons of Yosef his sons. It is that he is praying to the Creator that the 'Redeeming Angel' that looked out for Avraham, Yitzchok and himself, would also continue to look after these children of Yosef.
Yaacov wanted the angel to include Yosef's children in that protection that had come to the rest of the family as a result of their relationship to Avraham, Yitzchok and Yaacov. Since Yaacov didn't make a similar prayer for the other grandchildren who already existed, it must be assumed that Yaacov didn't have a question about them receiving that angelic protection. And this seems to follow the Targum Yonatan on Genesis 48:16:
יְהֵי רַעֲוָא קֳדָמָךְ דְמַלְאָכָא דְזַמִינְתָּא לִי לְמִפְרַק יָתִי
מִכָּל בִּישָׁא יְבָרֵךְ יַת טַלְיָיא וְיִתְקְרִי בְהוֹן שְׁמִי
וְשׁוּם אַבְהָתַי אַבְרָהָם וְיִצְחָק וְהֵיכְמָא דְכַוְורֵי יַמָא
סַגִי מִסְתְּגֵי בְמַיָא כְּדֵין בְּנוֹי דְיוֹסֵף יִתְקְפוּן לִסְגֵי
An angel behaves strictly according to the orders that it was given and doesn't diverge from that mission at all.
Taken in context, this would suggest that something about the specific status of Yosef's two sons was different from the rest of the family in Yaacov's view.
To understand this properly would require close examination of the relationship of this particular angel to the Avot and the details of how it was commanded to interact with Avraham, Yitzchok and Yaacov.