Transgender was a 'young' word back in the day when she would be dating and marrying, starting a family. Not a lot of people understood it, a lot of people still are confused about what all that the word entails. I believe she said that she still is sexually attracted to women. It is more or less realizing that you are born to the wrong body. It has nothing to do with sexual fantasy or anything like that. There are a lot of stigmas more so than being gay/lesbian attached to that word. So you have to imagine that there is also a lot of fear for someone to come to that conclusion as well.
There is no reason to limit yourself to what you want (within legal reason), based on who you think you are. She probably wanted a wife and children. So she got married and had them.
Now that being transgender is becoming a little bit more accepted.. with more people becoming more enlightened to that, then it is a better time. Also she is older, her children are for the most part grown up. So they can understand the transition. She would have had more to lose when they were younger, now they can make the decisions for themselves whether or not to accept and acknowledge the transformation she made. Sometimes people have to make decisions for themselves that are difficult and seem selfish, but if it is the difference between accepting yourself, not being afraid, and finally being happy... I don't think I could judge a person for that.
Timing makes people so critical and it is sad. There is no way to judge what she felt or when she felt it. So there is no way to know when the appropriate time for such decisions are to be made.