Thank you all so much. It's a relief to speak about it tbh - I thought I was getting over the jealousy of others thing, as I've felt much better recently, but it seems there are still challenges. It's true, I come from a family of high achievers: both my brothers went to Oxford and my mother made much of that (I went to teacher training college, which happened to be in Cambridge so she told people I was at Cambridge!!!)
Our daughters did well but didn't go to Oxbridge - though both got Masters degrees and the elder one has a Ph.D (despite that she's no ambitious high flyer - has a baby now and wants to spend special time enjoying him while she can, which is wonderful). I have always felt inadequate intellectually and in other ways, from early childhood, so yes, it does feel like having salt rubbed into my wounds! I have a congratulations card ready so I WILL send it, and I replied to my SIL. I know what I must do, and all your advice makes a lot of sense. I agree with all of it. I just hate feeling like this AGAIN, but...oh, well, I've done it before and I survived it, so I can do it again, I hope!
As to strategy....I have never managed to find one. I tend to want to run away from painful feelings, which I suspect isn't helpful, so I will try the crying-in-the-shower trick, BluBelle, buy a massive straw and suck very hard! 