In this video I show that Stephen did not quote from the LXX (it didn't exist).
The LXX revisers reading Acts 7:14, couldn't figure out to reconcile the #70 (Gen.46:27) and Stephens # of 75. So they did what all Alexandrians did, they changed the text!

In the LXX, in Gen.46:20 they add 5 names, the sons and grandsons of Manasseh and Ephraim, to make Gen.46:27 add up to 75! Presto-problem fixed by God's little helpers!
The problem is that Manasseh and Ephraim were still boys and could not have had children or grandchildren.
Moreover the LXX contradicts itself in Deut 10:22 by stating the correct # of 70 not 75.
So, again, we see that Origen had the NT in front of him when he edited the work that we know now as the LXX and changed the Hebrew to match the NT Greek, just like any Alexanrian will do to make it fit his view of what the text SHOULD say.
Below Scofield correctly states that the #75 comes from adding the 66 to the 9 wives of Jacobs sons.
There is no real contradiction. The "house of Jacob" numbered seventy but the "kindred" would include the wives of Jacob's sons.