Just because 2 things occur at the same time doesn't mean that one caused the other.
You've just been through a big change. To get there, you really had to put a lot of things on the back burner. You were extremely focused on what you had to focus on - your intense studies, the demands of 2 small children, and a part time job.
But other people had lives during those 3.5 years too! Is it possible that you weren't as present as they might have needed? Is it possible that they had to look elsewhere for some support while you were understandably inundated with your responsibilities and deadlines?
What was going on with your SIL during that time? She apparently was working very hard to get necessarily skills and make massive plans to start her own company. She's in this on her own. It's all consuming, and it's a professional and a financial risk. So that's where her focus is. Just because she's not focused on you does not mean that she is jealous of your success. Maybe she doesn't think you have time to support her now? You comment that she doesn't have kids, so does that mean you think her life isn't so hard? Do you feel she doesn't understand what you went through getting an advanced degree and raising kids? Maybe she doesn't think you understand what it's like to start a business.
So she missed a text message. So what? It says a lot that you have leapt to the conclusion that she is jealous or unsupportive of you. You're texting her! If you want to talk to her, make a real, human phone call.
And two acquaintances have found other things to occupy them? How close were you? Probably not very, if you call them "acquaintances." Maybe even because you call them "girls" - perhaps they find that term offensive and a sign of you feeling superior to them. Maybe they think YOU changed, while you think THEY changed. So, if you value their role in your life, reach out and make a phone call. Not a text. That's impersonal, and it says, "I don't have time or desire to talk, but I want credit for making the contact. Ball's in your court now." Maybe that bothers them.
And friendships ebb & flow, people come into our lives and leave our lives sometimes but not because of something anyone did.
It's better to assume there is something you need to do to make a connection than to try to assume reasons why someone else has failed to make a connection with you. If we women put barriers up (I work/you don't, I'm a SAHM/you're not, I got a Ph.D./you didn't...one stereotype after another), we create more divisions and lose the tremendous support that is available if we make the effort.