I had DS1 at 18 years old.
My entire pregnancy all I got was endless grief, snide comments and rejection from my supposed "family".
My dad (upon finding out the news) got drunk and called me a 2-bob whore and went for me and DS1's dad.
My mum refused to let me stay in the house and although she thankfully didn't throw me onto the street, she did find me a little flat down the road and then visited me ONCE throughout the rest of the pregnancy. Out of sight, out of mind.
My birth family just pretended I didn't exist. Extended family members were plain nasty. I had phone calls and remarks made to me in the street about how I should abort or give the baby up for adoption as I'd be a shit mum and wouldn't cope. Told that the baby would end up in care. Overheard comments such as "that poor kid doesn't stand a chance" or "she's ruined now." Seriously!
I was also excluded from all family parties and celebrations. I still am to this day! The guy that nonced me and my sister when we were teens is still invited though, in fact he's top of the table!!! All cut from the same cloth.
In fact, my then 14 year old sister was the ONLY person that EVER wished me congratulations on my first pregnancy. A CHILD saw through all the shit that the stuck up adults couldn't and was just excited to meet her nephew - a baby is a thing of joy and she was the only person to remember that. Whilst I agree that teen pregnancy is not something to aspire to, I was technically an adult and me and my then partner were working so it wasn't like we were reliant on benefits either (another jab thrown my way).
My DS1 is 15 now and the light of my life! Lovely, funny, kind, tall and handsome and on course for top marks in his exams. I took to motherhood like a duck to water and he was a contented kid right from the word go. Maybe I got lucky or maybe I wasn't as shit as they all said?! Anyway, I'm NC with most of my family now and much happier for it. And DC's still have a wonderful auntie in my sis! 
OP you sound like a wonderful mum and your dad sounds great too. I'm sorry your Dsis has been so awful - my own aunt was one of the most abusive during my pregnancy and I've not forgotten it. So if your Dsis thinks she can just step back into her nieces life once baby has been born she might have to think again. This is rejection and shaming of your DSD2 on a huge scale and pretty unforgivable in my book. Maybe she'll regret it in years to come but it'll be too late by then.
One thing though: the possibility has been raised that maybe she's marrying a controlling man who is pulling the strings here. If true, it may be worth someone like your mum or dad keeping an eye on that so that your Dsis knows she has a place to seek support if things turn really bad in the future. I know your sis may not deserve the support after her lack of here, but more for the sake of her own DD (who, let's hope doesn't get knocked up at 18 also)!
for you and DSD2 - so glad you have each other...x