Mtnman, sounds like a nice time at the lake. If you, and your boys, enjoy her company then invite her, but I don't think you should feel compelled to invite her all the time. It's for you. These outings probably lay a good foundation for any future relationship with the W. Who knows?
I stopped trying to get my W to pay more attention to our kids. I no longer intervene in her relationship with my girls. I don't get in the way, they need a mom. But I cannot make her or guilt her into doing things that I think she should be doing. I think we just need to let our MLC W's be what they are.