I would ask for a guest list first. She might not even invite the Aunt. Does she know the Aunt isn't invited into your home.
I understand you don't want to put her in an uncomfortable situation. WOuld it be possible to ignore the aunt or ask someone to be a buffer for you so she will leave you alone ? Maybe even ask to buffer to act as a bouncer who can ask the Aunt ot leave if she doesn't behave ?
Honestly, if it was me, and I know that some might consider this uncouth, but I would talk to the husband and ask him try to convince the mom to leave her Aunt off of the invite list.
She does know the Aunt isn't welcome in my home. She was present at the family function where the Aunt brought my husband to tears at the Thanksgiving dinner table by announcing what an unwanted and unloved child my husband was, and making light of all the abuse and neglect he suffered as a small child. She also read the email the Aunt wrote saying that my husband marrying me was proof of his incredibly low self esteem, because i'm so overweight and unattractive. That's just the tip of it. It might be petty, but I'm not willing to bend on this one. She is cruel and two faced, and simply unwelcome in my life and home. SIL's husband would absolutely tell her to not invite her, I just worry she would feel obligated to anyway. I don't want to put her in that position, and when the Aunt eventually found out, she would definitely stir up some trouble over it.
On the other hand, I'd feel like a jerk for not even offering. I'm the only family with a large house and love hosting parties, it would seem odd if I didn't offer.