The problem here of course is that it puts Julie in a poor light if she is hedging her bets as to the murder of Nevill and June,whom it appears from her witness statement she did very little to endorse,in contradistinction to the twins and Sheila. There's also Jeremy statement that his relationship with Julie had been shaky for some time and though there's not necessarily any contradiction here it does seem as if Jeremy was more serious about the murders than maybe Julie let on to others post event.
Problem for who?
Any of the following is possible:
1) Julie thought Jeremy was going to kill them and figured after he did so then he would marry her so she failed to discourage him.
2) Julie didn't expect Jeremy would really kill them but after he did so she expected he would marry her and thus stayed with him for a while but after a few weeks went by it was clear he wasn't going to renew their engagement the guilt was too much so she decided to cut bait
3) Julie didn't expect Jeremy would really kill them but after he did so she stuck by him because she loved him but after a few weeks went by she felt guilty and he was acting like a jerk to Colin and others and she couldn't take it anymore so she decided to cut bait.
4) Julie thought Jeremy was going to kill them and figured after he did so then he would marry her so she encouraged him to do it.
The only problem is 4 and that is a problem for Julie because it makes her a co-conspirator. It's only a problem though if Jeremy confesses and rats her out. So if 4 were true she actually would have a motive in keeping him from getting blamed so he didn't say anything against her.