If your partner is happy with her being there, by all means invite her. Many families these days struggle trying to reconcile earlier relationships, and if you can cement this one and keep it amicable, it will be to all your benefits later on. Family members now get their support from all different sources and can't change their earlier history, so those familities who are able to maintain amicable relationships usually reap dividends later on. Your partner's "ex" will probably appreciate the invitation. If she doesn't want to accept, she can think of an excuse, but I'm sure she will be pleased that you both think sufficiently kindly of her to want her to be included in your celebration. Similarly, if you can gently reciprocate in her family's affairs, it will make life between you all much more pleasant and trouble-free in the future.