The Reverse Sit can be an absolute nightmare when you're trying to retain your guard, work sweeps, or try to submit your opponent.
Avoid the Reverse sit can be difficult as well. It is very important that you have strong fundamentals and an understanding of body positioning in Half Guard.
The best defense for the Reverse sit is to have good body angle, your head/ear listening down by the belly button and frame the knee.
If you can place your head low enough on the stomach then you can avoid the crossface. By avoiding the crossface with your body positioning that opens your body up to frame the knee. Framing the knee avoids a lot (not all) of the Reverse Sit entries.
In the event that you got caught in the transition or out of position it's time to look at the most basic of reverses. This is not a high percentage reverse but I have used it a lot with success. I have a series of escapes that will depend on how their body weight is. Unfortunately there is no way to really show you the body weight distribution without actively practicing it with you. As with all of my moves you will need to rep this out to build an understanding of the timing and the body weight.
Most importantly you need to defend against this position because, in my opinion, this no longer a version of Half Guard and more of a Half Mount (I understand the literal position is still half guard, but it's such a terrible half guard that I don't think of it as neutral).