I completely understand why you feel sad about this. You do need to approach her about it. If you don't it will always be in the back of your mind and could end up negatively affecting your relationship in other ways. If you talk with her, I would do it face to face, and just say you were really sad that she didn't come to your shower and ask her why she wasn't able to? Especially after running into them at your FFIL's house, that's just not cool. If you are truly close your relationship should be able to withstand a conversation like this. And if she cares about you as much as you seem to care about her, she will want to make this right.