Hiya, please bare with me, I'm aware I may be over reacting & hormonal but ... a bit of back story so no drip feeding
I often used to look after my nephew as a baby, my brother works away and SIL worked shifts. I would help out where possible, do nursery runs (it's right next to my work).
SIl became SAHM for a while so for about 16 months I didn't babysit at all, but would still pop over to see SIL while my db was away.
SIL has a new job & I have a weeks holiday, nephew was poorly at the weekend so couldn't go to nursery but SIL didn't want to cancel her shift, I offered to help last minute.
Nephew is now 3, I haven't looked after him since he was a baby and don't have much experience with toddlers, all morning he tested boundaries - expected behaviour.
I went to the supermarket just to grab a couple bits for our lunch, took DN with me, walked past magazine aisle he asked for one and I said yes as a treat, explained we could sit and colour/do puzzles together after lunch.
next aisle... sweets, DN turned into devil child, demanded I bought him something, and it all escalated into ww3. I put the magazine back and dn proceeded to throw himself onto the floor, kicking screaming. I tried picking him up to get him out the shop & he started kicking and pushing me. Just at that moment SIL phones to check we are ok 
she could hear the commotion & I explained what was going on and that I needed to go.
I am 32 weeks pregnant with our 1st baby, and the reply I had from SIL was ' maybe you should have thought about having this kid if you can't handle a toddler for the day'.
The comment really hurt, we've been ttc for nearly 4 years & are more than excited about the imminent arrival, and secondly I was doing her a favour. I was kind of hoping for a few words of 'what to do' rather than a slating.
He had another once we were home as I told him not to keep pushing the Telly making it wobble, and he then smashed his toy he'd bought for the day into little bits. 
I am due to look after dn again Thursday but now I really don't want to.
So wibu to say no, and if not then any advice on dealing with these tantrums, they are beyond any kind of strop I've witnessed before.
It's not like I can even have a glass bottle of wine afterwards!