Hmmm, yeah sadly, when someone who has been childfree for some time has a baby, they and their childfree friends (or in your case - sister,) sometimes find their friendship falls off a cliff. I lost 3 or 4 friends when I had my DC, they avoided me like the plague and phased me out! I did make 3 or 4 new friends who I met through mum and baby groups though, so swings and roundabouts.
Some people who don't have children don't like to be around people who have children.
It's not a slight or a critisism, it's just how some people are. Some childfree are fine with people with children, (including people who have children after being childfree for a long time.) Some are not.
Never sure if it's that they find mums and babies boring, or if they're jealous and resentful, or if they just don't think you're a good fit anymore, because your priorities have changed. Whether anyone likes it or not, most mums will see their children as the centre of their universe, and some people can't handle that - or just CBA with it.
Sad that this is a sister who you were close to though @Travellingpantss Give her some space, and time, she will very likely come round, She has to surely?! Your babies are her family! (Your DD is her niece...) Reading between the lines, I think she may be a bit resentful.... She should be thrilled for you, and thrilled to have a new little niece. Her behaviour is odd IMO. (I mean to behave like this towards her own sister, and niece...)