Nanny kisses, when you say "but please be kind as I feel very low & unwanted atm x" I meant to say I really appreciate how you feel. I think Christmas is an especially sad time and feelings of unhappiness, loneliness, and generally being despondent are most prevalent at this time. The awful adverts of "families" having perfect Christmases, or going on wonderful family holidays or setting off happily to the Sales only enlarge those feelings of isolation and unhappiness in us. It is probably true that most people are feeling that anti-climax now, after the revels of Christmas and New Year, even those whose holiday period went reasonably well.
I can only say please do look after yourself. You may be a bit run down too, so some vitamins might help. But molly-coddle yourself with a nice cup of tea/coffee and a biscuit, treat yourself to a lazy day or a trip out or anything you want. Just be kind to yourself. Ignore those selfish people. They probably will learn their lesson one day. Try not to let the awful Sil's behaviour impinge on your feelings or thoughts because she is not worth it. Don't try to work out how she could behave like that because you won't understand, you are a reasonable and kind person who to put it simply plays fair, she is an unreasonable, manipulative and selfish person who uses other people so that she can get what she wants. You will never be able to understand how she could think it was ok to behave like that because you are a decent, kind and - well, frankly, normal person! She is one of those who does not give a fig about others and does not care about anyone but herself or her own daughter. These types, once you discover them, are best avoided as far as possible. Keep conversation to a minimum. Your replies to their questions are simple statements of your position, e.g. I give £50 to nieces. Whatever she says next, your reply is the same, same polite voice, same tone same words, "I give £50 to nieces". Or you may even cut back next time.... "As I gave you double my normal gift, that Is £100 last Christmas, I have had to cut back to £20 this time." It might not be a bad thing to let the niece know she received £100. It's such a lot for a niece! Even my children did not get that much!
Good luck, my love, don't let yourself get low, do something you enjoy if you can. Ignore the stupid people. My friend always said "Don't let the Burgers* get to you!" (*change the r for a g). ☕️?