Yes, it's definitely happening with SD13 full on.

SD13, DH and I were having dinner last night and DH and I were talking about plans for the following weekend when YSD would be at our home.  SD says, "But I won't be here, I'll be at mommy's."  We both said, "What are you talking about?"  Well, apparently BM has made plans not only this coming weekend (she has the SDs for Easter, no biggie) but also the following weekend, on DH's weekend. 

Classic move that's happened I think three clear times since November-ish.  BM doesn't contact DH for the schedule change, instead SD is made to be the bearer of the news and then DH cannot say "No" easily because now HE will be the bad guy.  It's a no-win. 

DH was visibly upset at this and asked SD why she wanted to do that and whether she'd rather be with us on one of the weekends; and that the every other weekend schedule should be followed.  She said no to Easter (obviously because that's a prime Disney BM day) but she did let on she was missing several things by being with BM the following weekend which she could do if she was at our home.  However, she was very, very reluctant to even try to think about saying no to her mom and she was upset DH was upset.  I told DH as quickly as possible (because he cannot make SD the determiner of this) "Hey, DH why don't you contact BM and let her know if she has SD for that extra weekend then you will take her for an extra weekend on a certain date in May."  Then steered the conversation to other things.

And now I am done - I always advise DH to remain calm, do not react to BM esp. in front of SD(s), and when emailing her remain cool, calm and facts only...because you have SD on my weekend X then I will be having her at my home on weekend Y to make up for it.  But he never does. 

Rope dropped because I have no say - it's sad to see this happening.