Babysitting my sister's three girls while pregnant, need advice!

To avoid babysitting costs my sister and I babysit each other's girls. I have a three year old, she has a 4 yo, 3yo, and almost 6 month old. I'm 27 weeks pregnant and quickly losing patience with them all, even my own daughter. We only work part time so we babysit two or three days a week each.

My sister's girls are very rambunctious compared to my daughter so it was an adjustment at first. My hormones have me raging at everything right now and I had very little patience before I was pregnant that I was finally getting under control until now.

How do you handle toddlers with a lot of energy and a strong will to do what they please no matter how much you ask?

The usual course I take for my daughter when she's misbehaving is to ask her nicely to stop what she's doing and tell her why she shouldn't do it, if it continues I tell her in a stern voice, then I raise my voice if the second option doesn't get her attention, and on rare occasions I have to resort to a time out or a swat on the butt if she's being particularly naughty and depending on how dangerous the particular action she was doing is. (Yes, I spank my child, one swat to get her attention, I feel a spank on the butt is far less of a risk than her potentially breaking her neck, cracking her skull, or being electrocuted.)

I just really need some advice right now on how to handle the situation without flying off the handle instantaneously.