So sorry to hear what you are going through.
Can I say that until you have got advice and got a good idea of your plans that you don't say anything else to him or your SIL ( assuming she's a SIL on his side) .. because if you start saying I'm doing this or If you don't Ill do that.. then he will be able to put barriers in the way of that. Better to just do.
Also. if you are leaving but need time, its better to keep the atmosphere a bit kinder for yourself and DC.. ie you are disengaging because there's no response when you do.. but just go about your business without making any annoucments etc.
When I heard that he was investing in property but not paying off your own mortgage it made me think that he thinks he's reducing the amount you would get from the sale of the house you live in. Ie interest only on the marital home.. money invested elsewhere. So I do think he may be trying to limit what you can take in a divorce, which is crap for your DC.
How does he get on with your DC. Would he want 50/50? or would he say that to avoid maintenance. It sounds like you have been the main caregiver all through. Your children won't forget that.
Do you want to go on this two week holiday? It can be so pressurised that my may feel you are about to burst.. and surrounded by his relatives... alll arguing his (unworthy) case. Would you be better off not going and working on your plan? Alternatively on holiday he has no excuse to say he's working.. but you would both be under the spotlight.
Whatever your decision is.. this time will pass and eventually you will be through this and on the other side of it. Get all the help and advice from professionals that you can and keep things undercover for the moment with relatives (on his side at least) Best of luck.