Trouble telling someone who's having trouble.

We want to announce our pregnancy at Christmas.... But my one sister in law and her husband can't conceive (so I've heard). They have been going to see a bunch of Doctors so I'm told by my other sister in law. She tries saying im good with no kids right now when I try asking her but I can tell she wants a baby. Well I guess my point is how do I tell her I'm pregnant with out hurting her feelings and being too overly excited when I tell her, cause I really don't want to hurt her. I realize I should know how to tell her since I also had troubles conceiving and when I heard my friends were pregnant I was upset. But I just have no clue.