I think it's wonderful that you're thinking carefully about this, and how it affects those you care about. As important a personal journey as this can be, gender is "only" one piece of the puzzle that fits together into ourselves. For me, it's been really important to think about the bigger picture, not just decide to transition because I know I ought to have a female body to match my brain. My goal is to be happier and to live a fulfilling life, and if transition would not improve my chances at that, I would not do it. For others transition may be more essential, but although I'm dysphoric, I know from experience that I *can* be pretty happy in the form I was born with. Not everyone can.
I worry a lot about how my transition will affect my own children, particularly the eldest, an 8-year-old boy. That's actually the biggest concern I have about this by quite a wide margin. After thinking a lot, talking to my wife (we are staying together), talking to my therapist, thinking some more, etc, I have decided that it will be ok. Children are flexible, and while I think it will be difficult for him, he will be ok. My wife and I are staying together, so regardless of my gender expression I can help give him a loving, supporting, safe home. He won't have a man in the house, but millions of kids do just fine that way, and he'll have two loving mothers who will do a fine job. One will even be able to teach him all the tricks guys need to know.
The way I like to look at it is that yes, this will be difficult, but that's not the same thing as bad. I don't accept "that's hard" as an excuse from my kids---I expect them to learn to do hard things. He (and his younger sisters) will get through this and they'll be all the stronger for it. He won't understand this now, but years from now, when he's faced with a difficult challenge, I think he'll remember my example of being honest and true to myself in the face of adversity. I don't know how I can teach him that if I hide from who I am.
Anyway, that's how I feel, but everyone's situation---external and internal---is different. I am not trying to tell you what to do, but I am hoping you'll at least consider that having an older family member come out as trans does not have to be a negative thing.