Few things. Where’s the BM & why isn’t she taking her daughter at all?

I’m assuming the 1 year old is also your BF’s child? If so, why is he only spending time & money on SD? I’d be upset too if my DH came home & showered the skids with gifts while ignoring our bio child.

What is he doing to support you while you’re at home with both kids during the week? Buying SD gifts isn’t enough, he still needs to parent her & suport you in parenting her when he’s not there if this is going to work (again, where is BM?).

Being pregnant is really hard, much harder than I anticipated, and hormones really suck. So give yourself a break for not quite being yourself or as energetic as everyone wishes you were right now. Your BF should be supporting you in every way he can, not calling you mean bc you’re taking issue with the Christmas happening each weekend. 

With his kids.??.  He can do that now !   He only a BF, this is a picture into your life.  BF dumping his kids on you and he’s playing at his aunts house.  Do three thing as fast as you can,  RUN,RUN,RUN.  It’s not going to get better 

You need to sit that guy of your's down and lay out reality. 

After a talk this next coming weekend, you take yourself off somewhere and do some 'me' time (even if that only means taking a walk or sitting in a cafe having coffee). Let him take care of the kids. Both kids are his and he obviously thins taking care of kids by one's self is apiece of cake as he is about to have #3 delivered and he believes it should be easy peasy for you, right? 

Spend some time this week researching daycare/pre-school. If you need financial assistance you might find something that does by income/sliding scale. If you were working and/or going to school the state might help with child care , but as a SAHM perhaps not. You need to have two or three afternoons a week where SD4 can go and be with kids her own age, wear herself out and have a good time while hopefully learning skills too. 

This isn't optional. If your BF has "stupid"money for weekends he has some means of financing a bit of a structured home life for his GF and kids. Go to a resale baby shop and see about a used double stroller. It'll get you and kids out of the house and out among other people. 

What happening with you in the home and all alone could very well be going on even with SD4 there all week. Being young and trapped in the house surrounded by demanding whining kiddies isn't easy. Your BF needs to be left with all his little darlings on a regular basis. It's his duty as their father... stop being so nice about it. 

It has been a MONTH since he wrecked the car. Is it in a repair shop? If not, WHY? If he has money to shower gifts every weekend, he has money for the car to be repaired. 

IMHO, you being pregnant and having a 4 and 1 year old at home without a vehicle is UNACCEPTABLE. 

Not only does he need to get your only car fixed, but he also NEEDS to start setting funds aside for a second vehicle. 

Have you looked into part time work from home? Perhaps for a few hours a night when the kid are sleeping? Amazon frequently has positions like that available. Not only will that give you contact with the outside world, but also help save for a second vehicle.

My BF I guess is just used to throwing all responsibilities on my when it comes to the kids so he doesn’t see why I’m having such a hard time controlling her during the week.

Don't blame your pregnancy or your hormones on this one. Your BF's ability to throwing ALL his responsibilities on you when it comes to the kids is what's affecting you. Of course he has no idea what you go through, he's not around during the week. He has a sweet deal if you ask me and its time he comes back home and handles his business as a father and a BF.

Your boyfriend needs to consider the health and well-being of his family that he is abandoning all week. The health and well-being of the overall family starts with the person in charge - YOU. Why is all this money being spent on SD over the weekend when he's home? He has TWO children, not one - not to mention another on the way. Why waste money while you are pregnant? Most people do the opposite. 

Our whole household was aware that while I was pregnant, money was being saved, not wasted, because we were going to have less income while I was on maternity leave. Any extra funds were spent on things for the upcoming baby or savings to help us get through one person being off work when the baby was little. 

Back to my point about YOUR health and well-being, though. This is essential to the happiness of your kids because you're the one taking care of them. He needs to be focusing his efforts on helping YOU get extra rest/time for yourself when he's home on the weekends. Why is this not happening? Tell him that money going towards SD needs to start: 

1. Being equally spent on BOTH kids or not spent at all because you are pregnant. My 18-month old would be pretty upset if toys were given to his older sister but never him. Your son is old enough to at least feel upset when new fun things are given to someone else all the time and not him. 
2. Being put towards a spa day or something similar to help you get some much-needed relaxation time. You need time for yourself, desperately, because spending all week isolated in a house with children is really, really hard. 
3. Being SAVED in an account for the upcoming baby that will cost you both a good amount of money lost in work wages/put towards diapers, wipes, etc. 

When I was a SAHM to my son (until he was 11 months old), my husband's time off was usually spent making my life easier/taking the kids while I took a break. When he was a SAHD (from 11 months til 18 months), the situation was the same for him. I even worked extra to save a few hundred dollars to send him on a boys weekend when I knew things were really rough for him.