This is very common behaviour for when a toddler is expecting a new sibling. They do sense that 'something' is changing, perhaps it is unconsious on the mother's behalf, I suspect you start to go a bit inwards in preparation for the birth.

I guess all you can do is give reassuring cuddles and sit together, maybe read a book together (one about sibling babies maybe?). If you partner or someone else close can give some extra attention to your toddler, that may help too.