As a newlywed who has had a rough time with her MIL, I applaud your effort to find a way to ease potential tensions! Honestly, as far as if it will help or not, I think it depends on the reasons things aren't great now. In my case, the only thing that was going to make it better with MIL was time--time to see that DH was truly invested, time to see that I could care for him, time to get used to the idea that her husband-wife-daughter-son family was moving on, grown up, and his family was shifting her into a grandmother role, just as her daughter's family was also doing... Finally, after a year of marriage, and 5 years together, she's figured out that she can count on me to get him to the hospital when necessary, that I'm capable of caring for him, and that *this* is his nuclear family now...
That being said, if it's just a case of not knowing each other well, and you have both wanted to go to DL, I'd say, "Go for it!" I would be careful to make it a relatively short trip, and do separate rooms, and consider if it would help or hurt to have your fiancée there. In our case, having DH there would have made things take even longer to get through, as she would be so focused on him (his comfort, his hunger level, his knee pain, etc) that we wouldn't get any talking done!