I agree with pps that your DD was ‘performing’ for her friend & the conversation didn’t descend into a spiteful MiL slagfest.
However, your reaction to her comments & your feelings are what matters.
You could have breezed in, joined in the joke or ignored what you overheard & been utterly fabulous but understandably your emotions - shock, anger, hurt, shame etc - wisely too you away from the scene.
This has hurt because you know at your core that you are doing too much, giving too much & you have been shown (or heard) that your DD & SiL take you for granted, will not speak up for you in the midst of friendly banter. Their lack of respect for you has been shown.
You work p/t in a demanding role, you do far far too much for your DD, your SIL because he has to do less to do for his family, & loads for your beloved GC.
It’s time to regroup & work out your new rules of engagement.
No more errands, no more favours.
Minimal ad hoc childcare - medical emergencies only.
Reduce routine childcare now.
The new term starts in a few weeks, so start as you mean to go on - tell them what you will & will not do.
Your DD & SIL need not know that you overheard them, because then it becomes all about them rather than what you need & want to do. It is enough that you want to do things differently - it is your life, it is your time & energy to use for your benefit & enjoyment.
What can they do for you?
Trade some favours, get them busy while you watch the kids.
DD does your shopping?
SiL does the garden twice a month?
Loads here on MN & pop over to Gransnet for a tales about Gparents being overwhelmed, stressed & exhausted to the point of detriment by looking after GC beyond what is reasonable.
Your accidental eavesdropping has done you a favour, a gentle wake-up call about stacking your priorities, commitments & boundaries to your needs & preferences.