Tell them now what your plans are. Write to your M in L and your Mum if necessary. Start with the lines: "this isn't open for discussion but we are changing our plans for 2016" ….. then state what you intend to do. When my Mum took up with a strange man a few years ago I couldn't stay in her house as he was sleeping in my Dad's bed

. So I wrote to DH's brother/wife and told them that we would be spending C.mas here - that was after 34 years of us travelling there

. We had the best 4 C.masses, me and He, ever. Then the man died ……….

His brother/wife didn't say anything ;-).
As for the locks: get them changed, it takes 1 phone call. When my Mum had someone enter her house when she didn't want them to: these were people she didn't know particularly well, who had a key

something Himself and I didn't get for years

: a 'phone call to the double glazing firm brought them out that afternoon.
If your Mum then keeps harping on about having her own key tell her quietly that your Life has changed, there will be 2 keys - 1 for your husband, 1 for yourself. End of. OK she'll kick up, simply laugh at her? with the words 'you really aren't listening to 'me' are you' ………. don't wait, get it over and done with.
OK so they *may* be hurt and upset …….. however, that is *your* take on their feelings, you really are NOT responsible for how they feel. By telling them about the change of plans now, enables them to make/not their own plans ;-). This is no longer about the two Mums but about a peaceful few days with your son and husband. If your husband keeps on, tell him you will be going away alone ………. if he really wants to have them 'as usual', then here's the shopping list!