Well, Matt, since you're still here and talking...
You've finally let you true emotions surface. 12 years is a long time to hold them in! Even 10, if NRE pushed your discomfort with FoL's poly identity (I'd call it identity, not "lifestyle") underground for a while. I'm American but I understand repressing emotions is a long held English tradition. So, that might have been quite hard to overcome. Maybe having sex with Si a few times triggered the opening. Ironic.
So, in a "mouse that roared" kind of way, you've opened the floodgates and a decade of resentment has surfaced. Now you're thinking in black and white terms. Burning all bridges. Cutting Si completely out of your lives, or divorce, ugly custody battles, 12 years of investment down the drain.
I find it almost funny you're saying you "might" be hurting FoL. I am sure she's devastated. She's said here she's barely holding on.
There is huge room for COMPROMISE here. In fact, healthy marriages are full of compromise. FoL and Si could never compromise on time management before because you deceitfully acted like you were fine with Si being around as much as she was. Now that they (finally) know you want more one on one time with FoL, and more family time without Si around, as adults, y'all COULD compromise. Other poly families here can and do do this, every day.
It hasn't occurred to you that if Si came around LESS, or if FoL took the kids and went to spend the night at Si's place (just because she is a "homebody" does not mean she couldn't COMPROMISE in this way), you'd have free time and less of Si, and your wife wouldnt have to choose between her 2 loves? Why so black and white, why so all or nothing? Time management is one of the first topics in Poly 101. Heck, even if you met and married a mono woman, you'd still need to compromise in some ways. Why not start now, with your love, your wife, the mother of your children?
I hope this idea is discussed in therapy (you may not like it, but boy... it's needed here), once the venting is out of the way.
I am not asking you to be the only one compromising. FoL and Si, maybe they arent comfortable with spending less time together. You're seeming to suggest that, but I wonder if it's really true.
Love withers under constraint; its very essence is liberty. It is compatible neither with envy, jealousy or fear. It is there most pure, perfect and unlimited when its votaries live in confidence, equality and unreserve. -- Shelley
There's no lying in polyamory!
I'm a 58 year old woman with 2 partners:
miss pixi, my live-in gf, 36 (together since Jan '09)
Ginger, bf, 61, married, lives nearby (together since Jan '12)