My immediate thought was also that the guy just wants to bang his sister-in-law and he can do it and get away with it because they want to ask her to be a surrogate. Especially since, like you guys already said, it's fucking free to use a turkey baster, you don't need to go to a fancy fertility clinic to shoot dick snot into a vagina because the person at the clinic will do the same damn thing as you can do at home with a turkey baster, only they'll charge thousands of dollars to do it. There is no magic in conceiving the "old-fashioned way" versus sucking semen out of a cup and blasting it into a woman. He just wants to stick his cock in his wife's sister. He'll probably be using her as a "surrogate" long after the potential loaf is born too.
Plus, if you can't afford a five-figure fertility treatment, your ass can't afford a kid either. Also, how do they know for sure the woman is infertile? The woman is always blamed for an inability to make a fetus when men are perfectly capable of being barren too. What if wanna-Duh has crippled swimmers? Does he get to fuck SIL for the rest of forever in an attempt to conceive?
Also, and this too has been pointed out, the kid wouldn't be the wanna-Moo's kid if Duh and SIL conceive it. Wanna-Moo would be the child's aunt, and what if the sister-in-law decides she wants to keep the kid when it's born, or wants to be its mother three years in? Wanna-Moo will have no rights at all and the legal aspect of this bullshit will get ugly, especially if they just go about barebacking and no contracts are signed.
If they wanted to truly use the SIL as a surrogate, they would be using Duh's sperm and Moo's eggs and placing the fertilized embryo in the SIL's uterus so it's fully the couple's biological child, but it's just being incubated by someone else.
Wanna-Moo has every right to be upset because Duh is saying pretty blatantly that he wants to have a child with her sister and not with her. I'm sure adoption isn't an option, given how stingy Duh appears to be, and also if they're going to such great lengths to have a biological/semi-biological child, they probably figure a second-hand child won't be good enough.
Such a bad idea all around. I hope this woman gets away from this guy who blames her for her infertility and he can go balls deep with his in-law without extra steps.