Firstly, congratulations!
To add another perspective that I haven’t seen so far expressed on this thread, I wonder if, because your parents know you and your capabilities, that this is less of a big deal to them because they knew you would get there at some point?
I presume you are in your 30s (based on the fact your sister is pregnant).
If your are on a fairly defined career path (lawyer/teacher/police/any number of other things) and your parents know this, then they will be expecting that at some point you will get to where you are. They are of course pleased, but had the confidence you would get there at some point, and don’t have the finer knowledge that you have to understand how fantastic it is that you have done it at this particular point.
In addition, given they are 20-30 years older, they may have friends who are on even higher rungs, and have expectations you may get even higher.
For example, when I was 30-35, getting to partnership in a law firm was something lots of my friends were aiming at. Some got it sooner than others, and it is always a massive achievement.
But, 15 years later, most/all people who wanted it have got there. And I couldn’t tell you which ones got it first.
I don’t think there is anything wrong with suggesting you go out to celebrate if you would like that. Sometimes people need nudging to understand how much something means to someone else.