Due to time constraints and a bad back, this will be fairly short, but we'll get right to it.
L1: Dear LW1, the solution to your dilemma depends largely on the size of your wife. If your wife if a large woman, you can instance her desire, consciously expressed or not, for you to gain weight so that the two of you can be bookends, a popular look among many couples. If your wife is petite, you can express to your sister how much you envy her freedom to eat as she pleases and not to have to work out more than she chooses. Even if your wife is exactly your size, if she has femme tendencies, you can envy that your sister is more butch than you are, or vice versa. The idea is that there is sure to be some way to make it appear to your sister that you think her grass is greener than yours. That should be enough to quell this outbreak of rivalry.
L2: It was very noble of you, LW2, to become pregnant at 17 in order to bear a child for a male couple. Naturally, assuming your nobility received the lauds it deserved from your friends and relations, your sister might resent this. By the way, if you later reveal that you were both up for the role of Womb For Rent and you were chosen over her, then you are guilty of Lying By Omission. There is also the possibility that your sister feared you might be turning straight. It is one thing for people to accept and even approve of heterosexuality in others at a safe distance, but not everyone is comfortable with it in her own home. And it must be particularly difficult for a twin, who might have had a crisis of questioning her own sexuality. We well recall the recent scientific studies showing that identical twins in particular are far more likely to have identical sexual orientations than fraternal twins or other siblings. It's quite natural that her searching her soul for latent signs of heterosexuality might have contributed to her general unpleasantness. As for what to do about it now, I'd suggest matchmaking. It means you will come in for some unpleasantness should things turn sour, but her seeing the situation as being chosen in preference to you will do much to heal a bruised ego.
L3: Now this one is fairly easy, LW3. Your boyfriend's twin is an Angry Breeder Type. In fact, does he actually use the term Breeder when self-presents, in order to reclaim the slur? Whether he does or not, it seems clear that he resents the superiour attention and approval lavished on your boyfriend for having the "right" sort of relationship. And with cause, LW3, with cause. Unpack your privilege, please, and you will see that it must be particularly galling to those of the straight persuasion if everything they do never measures up to the lesser accomplishments of a gay sibling. Not that your boyfriend is necessarily less acoomplished than his twin, but it is common. Those straights who feel how society judges them often go through a stage of acquiring extra accomplishments with the idea that people will view those as a sort of compensation. It rarely works that way.
The Prudecutor actually almost gets this one right. She notes that most people don't have delightful relationships with each and every member of a spouse's family. And that this one particular family member makes you rethink the question of marriage makes me wonder. Are you sure there's not a little element of heterophobia at the root of your reluctance? Yes, it's unpleasant to be around people who don't try to get along, but they do have some decent cause for being antisocial, even if insufficient cause for so much action as the Angry Breeder Type often takes. Using the F word was highly inappropriate, but it is not fair to pretend that a member of an oppressed minority using the F word is every bit as bad as a member of the oppressive majority using the B word to hurt and wound. Given how society raises "normal" people to think of straights as just a little less than ourselves, it's quite understandable if you've absorbed a few of these messages and aren't comfortable yet with having a straight person so close in your husband's family tree. Maybe a little visit to PFFS is in order for your 101 work, a role that Parents, Friends and Families of Straights has done to general admiration for some time.
L4: It is admirable to want to jump in and advocate for your mother-in-law, but please tone down your White Knight inclinations. You presumably don't really know anywhere near so much as your husband's family does about straightness and how your mother-in-law's mental health might have been impacted by negative life experiences related to her open heterosexuality. Not all legally married opposite-sex couples have found everyone to be accepting of them. It is admirable of you to want to change that, as you doubtless do, and you quite naturally might feel that her family's treatment of her is not right or proper. But you are not the expert here. Her family are the experts on how to help her get through her life. There are few things more annoying to members of a minority than some well-meaning do-gooder riding in on the High Horse of Privilege claiming, without any knowledge of the particulars of the case, to hold the key to Enlightenment.