I'm not sure who Jenna is in your playthrough, but if she's one of your sisters or your mother, you should be able to get scenes with them together, as well as a combined wedding.
I am surprised you don't know who Jenna is, as I used the default names you put in the game! Susan for Mother, Liza, the oldest Sister, Jenna the middle Sister and Rachael the youngest. I was with Liza and Jenna, and as we were all marrying each other, I should have seen Jenna and Liza kissing at the wedding ceremony (they had already had sex with each other), I should also have seen a three-way scene with me Liza and Jenna, but what I got was a scene with Jenna, and then a separate scene with Liza, who actually comes to the beach after I made love to Jenna saying she was "lonely at the apartment"!! I felt that was sad, I didn't feel I had a double marriage, but two individual marriages, making love to each wife totally separately, when I felt we should all been together. To say the ending was a disappoint is an understatement.
Also, if the youngest, Rachael is dateable, i'd love to know how, because I feel you have no chance with her in the game.
The middle sister's default name is Bella. There are several ending varieties after the marriage and a Bella/Liza/MC threesome is one of them, there quite a lot more (Bella/Liza/Rachel/Susan, Liza/Rachel/Susan, Rachel/Susan to name a few, those also come with pregnant and non-pregnant branches).
Rachel is certainly dateable, but the trick is to get a relationship with her early (this applies to all of the sisters, to be honest).
Look, I have been complaining a lot, and you have been kind enough to reply. I will start the game again and see what happens. But can I totally compliment you for how each character acts with you, their personalities shine through, and they come across as real people. I don't know how many you've been with, but I am amazed at these marvellous character conversations!