Honestly, at 8 months I don't think he's old enough to feel 'jealous' of his sister's dummy? Does she have it in the daytime? You could wean it in 4 days and by the time she comes to stay again he could be sleeping without it.
Alternatively, you could try a different approach, which is to make a 'dummy blankie'. Get a muslin cloth and with a few big secure stitches, sew on about 6 dummies, all over it. During his A time, do lots of practice finding the cloth and putting a dummy in. Then start leaving it in his cot with him for naps/nights. The thing is, you can't use PUPD if the dummy is a prop - if he can control it himself, then you have that sleep training tool at your disposal.