Do you want us to agree with you, do you want to vent to the world - or do you want to talk about reasons why she might say things like that?
You want her to agree with you and validate your anger.
She considers that there could be other influences contributing to a situation - such as somebody's mental health making them less likely to make a correct decision. Doesn't change the fact that what they did was in opposition to the social distancing advice, but it might go some way to explain why they made shit choices.
She doesn't get angry at your sister for potentially putting her at risk - because getting angry with her wouldn't help the situation or change the fact that she's already been there/she doesn't want you falling out with your sister over it on her behalf.
Even though she hates your ex, she tries to consider there are explanations or factors around things he's done - it would be easier to say 'He's a bastard' and leave you even more wound up about him than you were already, chucking petrol onto a fire.
You interpret her as being aggressive and making excuses - could it be that it's not excuses but possible explanations? Because raging at the world isn't good for you? She doesn't like you being unhappy and angry and tries to get you to slow down and think about reasons being more complex than them simply all being cunts all of the time because being that angry?
Looking for reasons is not the same as making excuses. It is not the same as saying you're wrong. But if you tend to call her when you're absolutely furious about something and then get angry with her because she won't engage in winding you up about the initial subject even more, you're possibly calling her at the wrong time. Wait ten minutes. Then call. Or call somebody else who enjoys stoking up your rage and save calling your Mum for when you want a calmer conversation.
CBT and counselling can be useful in helping people deal with ongoing stressful situations, such as dealing with arsehole exes or instantly feeling attacked if somebody doesn't agree with you completely and immediately - I wonder whether your ex demanded that total agreement and would be threatening or physically abusive if you disagreed with him or tried to tell him why you had done/not done something? 'The washing up hasn't been done' 'I've just got in from work/child fell over and needed a plaster and cuddle' 'Don't give me bullshit excuses, you're just an ugly, lazy bitch and you make me sick' - type of thing?
It's frustrating, but is she really pathetic - or are you saying words your ex would use for you?