Separation start date can be backdated but provided that both are agreeable. In your sister's case, since the youngest child is only six months old, she could at most backdate it to 15 months ago and that would be quite a stretch. Besides, it won't look like her husband would agree to that. Therefore, it would be best for her to be patient and wait out that three to four years for a proper separation to take place.
If she wants to quicken her divorce proceedings a little, she can consider citing unreasonable behaviour. But be prepared for some resistance from him and it may drag for some months and incur further legal costs.
As for adultery, the burden of proof is much higher as pointed out by others, which is why it is not even cited as the reason for some divorce cases even though in reality they could be so. In the absence of direct proofs, suggestive SMS messages, email or photos are circumstantial evidences that have to be accompanied by confession from the spouse and his/her co-witness (third party) be named (may be called to the witness stand). It is financially costly to prove adultery and at the end of the day, you get what others get, which is still a piece of divorce certificate.
So, as you could see, there are pros and cons in whatever ground the divorce application is based on. There are no winners in divorce but there need not be losers either.
Your sister wants a divorce, not an ugly divorce. Hope you could guide her along well.
