Congratulations on your pregnancy.
I'm sorry your family are struggling with how to deal with it, given your sister's fertility issues. But perhaps I can give you my thoughts as someone who had fertility struggles, treatment and now has children?
When I couldn't conceive, it became my whole world. I know it shouldn't have. I had a fabulous DH, a good job, great friends....but I felt as if I were being eaten inside out by a biological sickness. Being unable to have children was everything. It didn't just define then; it felt it would define my whole life, so it defined me.
Each friend who got pregnancy, I struggled inside. I tried never to show it but would be eaten up. Even these days, when I have children, I slightly dread and get jealous of pregnancy announcements. It's fine when they are born and become children.That period just messed me up so badly.
If my lovely younger brother had conceived before me, I would have felt so angry and jealous. It would have felt like not only was I useless and barren, but even the natural order of our ages had fallen in the face of my inability to have children.
So I have utmost sympathy for your sister. Before I struggled to have kids, I remember thinking a friend who was struggling to conceive and charting her temps was crazy. Why didn't she just have sex and wait till it happened? I don't think you can appreciate that horrible legacy of infertility unless you've been there. It's not rational.
I also think it is very hard as the parent of a child going through hell. Your mum struggled to have children too so she relates to her own precious first born girl. She thinks you are lucky and pregnant so don't need her as much as her older child. She sees your sister hurting and that hurts her. Ditto your dad.
Having said that, of course you absolutely deserve for your parents to be happy for you. But now as the parents of two children, I would feel awful if my beloved second baby was able to conceive when my first wanted that so badly. So I do think it's a difficult position. You want to protect your children even in their thirties.
I would try to swallow your disappointment in your parents for now. It is a wonderfully happy time and you can indulge that pregnancy joy with your inlaws. Hopefully when the baby arrives and everyone gets to see this amazing new human being joining the family, things will begin to feel much better again.
Sorry again you are feeling unsupported - this is not about you and your pregnancy is a wonderful thing. Hope it works itself out.