You will not be hurting her if you go - but you will deprive yourself and your supportive DH from the fun of Christmas activities with the GC - a chance that will not come again.
Offer to take Mum with you both in the caravan! - I am sure she will not want that; and then the decision to be left alone will have been hers.
It's not about being a "wuss" but about pandering to and encouraging your Mum's manipulative behaviour, which is good for neither you, your DH, your DD, your DGD or indeed for her.
Let her "seriously miss" your sister - you will seriously miss out if you do not take this opportunity that is being offered.
At what point will it be the right one to break out of this pathological cycle? - maybe it might be a turning point for an improvement in your own health - could this be the trigger that you need to move forward from patterns of a lifetime that have brought you no joy and injured your well-being? There has to be a time that this happens or you will be like this till Mum dies. Only you can choose the moment - this seems like as good a one as any, when you have spent the last couple of weeks being side-lined and insulted with the collusion of your Mum.
Can you enlist your DD to say to Mum that she desperately wants you to be there and that she is SURE that there will be no objection?
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. The children have several Christmas and out of school events this weekend. Ideally we'd take the caravan and stay up there as it will be cheaper than using diesel driving there and back several times. Unfortunately I know my mam will be upset because my sister has left and I don't think I have it in me to leave her alone all weekend.
As I was only informed yesterday it was obvious that I was an afterthought. I told my that I'd said DH was waiting but the sister hadn't listened to me. Mum said that she hadn't heard because she was talking to someone else. 