We had an outdoor party every year when my kids were younger and the forecast never seemed to work out as bad as you expect. You will find that the bouncy castle should be supplied with a rain cover - check with the supplier - you only then need to work out how to get them from the door to the castle without getting wet. What we did was beg and borrow awnings/gazebos and use one as a cover for the kids to get to the castle and the rest as cover for the patio area and went ahead anyway - adults will enteratin themselves and will be busy supervising kids anyway - kids will be happy enough jumping about on the castle and will really need very little else - if you can get one room in the house reasonably clear and fill it with balloons that'll occupy them for ages. Just remind everyone to dress warmly and it will all work out fine. Stick the sand pit under cover and that will be plenty to entertain them.
As for games to play with them - I would forget it for two year olds when there is a bouncy castle there, they wont settle to anything - better just to have one idiot, oops volunteer, run about in the garden and let them chase him about - remember they will be hyped up and full of energy!