Cousiblings?

You are fairly close to the EXACT reasoning, IF it can be classed as REASONING that is) that both Christianity and Pedigreed Dog Breeding Associations have been touting for decades and in the case of Christianity, for centuries.
That ideology has been, "That, " and PLEASE feel more than free and welcome to laugh as long and hards as like, " should a female of breedable age be mated with with another male OR, a male of questionable pedigree THEN any FUTURE issues she produces SHALL be so tainted by her past mis-mating that they shall always be completely IMPURE."

I was told it was common and encouraged in the Bible but evidently that is only true if the widow is childless. If the widow has children which my mother-in-law did, it is not condoned in the Bible.It is definitely a way to keep the husbands wealth within the same family if he has any.

[en.m.wikipedia.org].

In fact it is Demanded in several places in the Bible. One example: Onan was killed because he was forced to marry his brother's widow & "spilled his seed" rather than possibly impregnate her.
And yes, the word "onanism" comes directly from that story.

@AnneWimsey Here you go, I looked up the actual biblical law, and it is plain that marriage between some "in laws" is forbiden.

The incestuous unions prohibited in the Law (some were legitimate at an earlier age in Israel) are those of son and mother, of a man with the wife of his father (Lv 18.8; Dt 27.20) and with the mother of his wife (Dt 27.23), of a man with his granddaughter or his wife's daughter or granddaughter (Lv 18.10, 17), of a man with his sister or half-sister (Lv 18.9; Dt 27.22; see, however, Gn 20.12), of a nephew with his aunt (Lv 18.12–14; cf. Ex 6.20), of a man with his daughter-in-law or with his sister-in-law (Lv 18.15, 16; 20.21); levirate marriage is an exception (Dt 25.5–10). Also forbidden was marriage to two sisters at the same time (Lv 18.18), although formerly it had been allowed (Gn 29.27–28). Penalties for incest were death (Lv 20.11–17), excommunication (Lv 18.29), and being cursed (Dt 27.20, 22–23), e.g., by being childless (Lv 20.21).

@AnneWimsey, @Lorajay Here you go, I looked up the actual biblical law, and it is plain that marriage between some "in laws" is forbiden.

The incestuous unions prohibited in the Law (some were legitimate at an earlier age in Israel) are those of son and mother, of a man with the wife of his father (Lv 18.8; Dt 27.20) and with the mother of his wife (Dt 27.23), of a man with his granddaughter or his wife's daughter or granddaughter (Lv 18.10, 17), of a man with his sister or half-sister (Lv 18.9; Dt 27.22; see, however, Gn 20.12), of a nephew with his aunt (Lv 18.12–14; cf. Ex 6.20), of a man with his daughter-in-law or with his sister-in-law (Lv 18.15, 16; 20.21); levirate marriage is an exception (Dt 25.5–10). Also forbidden was marriage to two sisters at the same time (Lv 18.18), although formerly it had been allowed (Gn 29.27–28). Penalties for incest were death (Lv 20.11–17), excommunication (Lv 18.29), and being cursed (Dt 27.20, 22–23), e.g., by being childless (Lv 20.21)...

@Fernapple yes, there are those things in the Babble, and they are directly contradicted in other places (imagine that!), like the story of Onan......or Jonathan marrying 2 sisters(Rachel & Leah) but not at the same time...and he was basically extorted into marrying Leah!
In many cases in actual fact, if a widow wasn't taken into a brother-in-laws home, she starved or went to a city to be a whore, not many options!!!.....and marrying her maintained her status, how could you reduce your brother's widow to "concubine"? Not cool!!!