Thanks both for your replies.

They are a large family, so drawing the line now is a good idea. They get a joint present for Christmas, plus a small gift for his two children and her son. Then there's the new baby due in March. The birthday presents started when my brother met her mother, long before the latest partner came on the scene.

She was difficult to get on with at the beginning, but has mellowed over the years. She now has a good standard of living but there's a certain degree of arrogance. There's a different moral compass for many things. We've been side-lined by my brother in favour of his new family (my mother's words). Yes, it's a social obligation.

Sending a card is more gentle/measured than just doing nothing as that would be viewed as spiteful. There will be a different version of the truth circulating anyway