Update...
I wanted to thank everyone for their input, and it helps to have an unbiased reality check that tells me when I'm being too judgmental, even if I don't want to hear it!

Well, the cabin issue ended up being a non-issue. SIL gave some lame excuse about her vehicle not working as a reason she couldn't go (even though her new bf had perfectly working vehicles which she had been using). Well, I recently found out the real reason they didn't go on the bike trip.
SIL is pg and due in March!

This was a shock to both SIL and her bf because A) she had fertility probs with xBIL and had given up on getting pg and B) bf had had a vasectomy 16 years ago. They are getting married in a few weeks.
So, I now understand why SIL seemed to be "pushing" her new bf on us, as she plans on his being a permanent part of her life now. So, I have moved beyond judgment and am going to enjoy having a new nephew or niece to love. The odds against this conception seem to be so great that I just have to believe this baby is meant to be here, even though on the surface the situation seems far from ideal. And, hey, my boys will get another cousin to play with, which I did not think would happen.
Now I am just praying dn will make the adjustment well, and her feelings and needs will be honored. She loves the soon-to-be new BIL (and truthfully he seem to give her more attention than either of her parents), but still a lot has changed for her in a short time.