I think you were not out of line, but your supervisor was. Just my opinion. I'm sure that you meant no harm. It wasn't like you got up in the woman's face and said, "hey, my FRIEND from my old department just had TWINS. She still has BOTH of hers. Aren't you JEALOUS?"... or something wacked out like that. You made a plain truthful statement that was not swayed one iota by the woman's personal experience.
Yes, it obviously upset the woman. And yes, rightfully (she did lose one of her twins and will never "get over" that loss). However, her feelings should have no correlation to YOU. She will encounter people the rest of her life that have healthy babies and some of them might even be twins. This is something that SHE is going to have to deal with. She may need counseling to get through it. She may need more time to learn to accept it and how to respond (or how NOT to respond) in public when she is confronted with something that makes her uncomfortable or sad. But those are HER issues, not yours, or anyone else's.
Now, I know that sounds harsh. But that is the reality. If she isn't able to function outside of a cocoon where no one speaks "the T word" then perhaps she should do something about that. Whether that is counseling, or taking more time off or whatever, I can't say, and neither can anyone else. But it is HER responsibility to make the changes she needs to make if she wants to avoid hearing certain things or learn to deal with/manage her emotions if she does. No one else's.
I, like you, understand her very real emotions, and that NO ONE EVER "gets over" the loss of a child, and having her son probably is a constant reminder to her also. So is her SON going to feel guilty his entire life b/c his mother makes him feel guilty about the loss of his sister? That is just unhealthy. It's been a year. It appears that she needs to take more pro-active steps to deal with her pain. Having a supervisor speak to officemates about their 'insensitive' language is not the way to deal with it. It's a way to avoid it completely. And it isn't fair.
However, I think it is probably more likely that she didn't "go to" her supervisor and ask they speak to you. She probably was upset, and needed some time (either to leave work, or be excused for 30 minutes to compose herself or something) and spoke to her supervisor to get approval for that time. Then the supervisor asked why and she explained. I doubt she went to higher ups and said "Amanda said ___ can you make her stop?" So the problem for YOU would be the supervisor's reaction, which is what you stated in your post. That your supervisor was out of line "correcting" you for saying "the T word". And I agree with that. They shouldn't have said anything to you.
As far as what to do, I'm not sure. It may never come up again in the future (supervisor may realize it was inappropriate to "correct" you or realize they can't enforce anything anyway). If you bring it back up to them, you make force an issue where there otherwise wouldn't be one (for management anyway). So as far as advice, I'm not sure what I would recommend.
I certainly would not "take it out" on the poor woman who lost her daughter, though. Cold shoulder, or other unspoken ways. I doubt it was her fault. I think it was purely management being "overprotective" of this lady, and they were out of line.
If you did want to press the issue, go ask your supervisor, in a nice concerned sort of way, being that you ARE concerned, what would the consequences be if you DID happen to mention the "T word" in a conversation, or she overhears you on the phone with someone and it is said, or you say the "baby girl" word or whatever.