As has already been suggested talk to your psychologist.
They will help you to deal with this situation and letting them know what you are experiencing NOW will help them, help you.
Any therapy is Personal, and you have The Right to Have Your Therapy.
You have the right to share it with someone IF you want -Be Very Careful about sharing any insights with others.
Relationships change but any information/insights you have shared may be remembered. It's a bit like be careful what you put on the web it may come back and bite you.
So you are being wise and self caring by Not sharing your therapeutic experiences with anyone you don't feel you have a safe and caring relationship with.
On to how to deflect or better still STOP them asking you, your psychologist really will help.
However, there is nothing wrong with putting your hand up in the Stop sign and Stating "I can't do this right now".
Better still is the hand up Stop and I Won't Do this. I Won't answer your questions.
You can soften the "Stop" with the already good suggestion "It's been difficult enough to get through the appointment, I can't do it twice".
Then firm up with "I won't do it twice. I won't put myself thought it again now".
Please, please, please use the words Stop and Won't (Will Not) preferably at the start and end of the discourse.
If that isn't working you can try the following deflection techniques. I know these have worked for others in similar situations to yours.
I have suggested to clients that they match their questioners ( interrogators) questions by being somewhat rude.
The questions you are being asked are very personal and it is very rude and damaging to be asking those questions so come back in kind.
As an answer to their questions ask "How much have you got in your bank?" We know it's none of your business - that's the point.
Really provocative is to ask questions about their sex life - anything you judge will be shocking. It is no one else's business - that's the point.
I'll probably get ripped at for suggesting this but I know it works so I'll take a bashing if it's coming.
The aim is to make them back off. Also to support your statement that You Have The Right to Privacy in areas of your life just as much as they do.
So a statement of I don't ask you about your finance, your sex life (or whatever touchy area you know about) followed by Stop asking me about ... my personal therapy, thoughts, feelings, hurts etc.
It's ok for them not to answer you, or for them to even get angry unless anger from your Mum or MIL is likely to result in you being fully attacked.
If an attack is likely you know how best to use any advice you've received to Keep Yourself Safe.
Just to be clear. You haven't said anything about being attacked and I'm not trying to suggest anything like that but I don't want you to come to any harm. I'm not there, I don't know what your life is like, just stay safe.
If you are not likely to be attacked let them be angry IF you can cope with it. Anger will most probably come as a Verbal Attack but you can get a lot of information from standing and listening to an angry person shouting.
Someone being angry at you and not getting a Fear response from you really throws them off their game.
And you are in a Game, a Power struggle, and by getting help your are starting to Win and that will not be welcomes.
No wonder your Mum wants to know what is happening in your therapy, she needs to have her game plan in place. By the way I am not suggesting this is at the level on conciseness. I doubt it is but never the less you are in for a battle.
The battle is for your freedom to be You. Fully you, strong you who is the centre of your world. Always a battle worth fighting but never an easy fight. Take as much help as you can but only that which really is helpful.
If anyone without good reason asks anyone for information they do not want to share and that person will not listen to NO then on their own head be it.
Being related, part of your past, your present and possibly your future, is not a good reason for sharing information you want to keep private.
So you protect your self, your feelings, the work you are doing in therapy and Never doubt You Have The Right to do that.
Wishing you all the best for a good outcome.