A young woman married her friend to help her out. She's not actually a lesbian but felt they didn't have any other choice. Kind of like the movie I now Pronounce you Chuck and Larry. They both want a baby so they ask the brother to get them pregnant. He's nervous but all too eager to help out.
Why Sis and Friend got married - Possibly as a condition in a will, Friend had to be married by a certain birthday in order to inherit millions of Dollars/Euros/Pounds.
That date was within just a few days and Friend asked Sis since Friend felt Sis would be the best available choice.
Friend would have asked Brother to marry her, but he would need his parents permission since he was under 18, and Parents likely would have refused to consider the idea.
Divorce before a certain number of years unless the husband spent time in jail for abusing Friend would have meant the loss of millions.
Friend also had to have a baby by a certain deadline to receive another part of the estate.
I can see several possibilities of the type of relationship Brother might have with Sis and Friend before they ask him for a baby.
1 - Brother has a crush on Friend (and possibly Sis) but thinks its a secret. He initially gets asked to get Friend pregnant, then Sis asks him to repeat that with her.
2 - Brother had a sexual relationship with Sis prior to her marriage. It might have continued. Sis and Friend ask him to gt Friend pregnant - Sis stops her birth control without telling Brother.
3 - Sis, Brother and Friend have an ongoing sexual relationship and the two young women ask Brother to father their babies.
4 - Sis, Brother and Friend have an ongoing sexual relationship. The two young women surprise Brother with the news they both will be having his babies. Possibly later revealed to be a way to see how he might react to getting them pregnant.
Just a thought of mine:: How about they all three have a party of their own after the marriage and he beds them both and from there on out continues to do so until both wind up pregnant.
How about a rich Uncle puts in his will that the first one to give birth will inherit the fortune.
or they make a bet - to see which one can give birth first/make a pact to raise the babies together like brothers/sister. Some girls do that in high school.