My husband has been acting a bit weird latley he sleeps all day as he works nights I'm out at work, in the last few months my sister seems to be tking an interest in what we do and what he does are the having an affair

Sounds to me like you have a very nosy sister that needs to learn to deal with her own life and stay out of yours and your husbands.

Talk to her. Ask her why she's so interested in what your husband does.

There is no way we can tell you if they are having an affair. I'd suggest talking to your husband as well - why don't you try spending some time together and reconnecting as a couple?