I think your sister and your families attitude is very wrong.
However, the nasty nature of some of the comments and the ill informed perspective leaves a bad taste in my mouth.
First, the use of a (consenting and willing) male donor with the sister in laws egg is far from uncommon, eitheramongst lgbt or straight infertile couples. Its not incest. It works fine in some families, but not in others.
Second, there is no such thing as anonymous sperm these days. All donors in the uk are traceable when offspring reach 18. As a consequence, numbers have plummeted and it is very difficult, as well as expensive, to get sperm from someone who isn't known to you andewilling to enter a private arrangement.
Third, the couple are in a civil partnership, so those of you who have used girlfriend, please don't again. It isn't accurate.
Fourth, to stooshe and anyone upthread who has done similar, speculating who is dominant in a relationship is generally not on, and always has a rather nasty and prurient undertone in reference to gay couples (I bet I know who is the MAN if you know what Imean, nudge nudge wink wink, chortle - the seventies are over folks).
Fifth, if a man donates sperm to a civilly partnered couple and both are put down as parents, no he won't be chased for it.
None of these points take away from the fact that a)the brother shouldnt be leaned on and b) the sister and the parents sound pretty horrible about this, but please guys, lets discuss this accurately and respectfully. Because all of the misconceptions I've referred to above are used all the time in relation to perfectly lovely and kind lesbian couples, and using them anywhere perpetuates it and that really isn't on.