D.K.
When she offers unsolicited advice to you, always say "well what works for one child may not another" or "we all have different ways of parenting" or even "well my daughter isn't this way or we do things differently"..
Who cares if she is that vindictive to hold back visiting with her son because you stand up for yourself! UGH! Sorry but you are way more diplomatic then I would be, hee hee. I understand with family you have to bite your lip though.
As far as the dr visits, unfortunately it is her call. However her not vaccinating can cause alarms and bring in unneccesary exposure to your child. You could approach it for a while with printing out a chart for vaccines, explain that you just want to protect your child and make sure when they play together they are both on the same page with vaccines. A child at that age can be a carrier without even knowing it! With whopping cough and measles/mumps back in circulation you can bring it up to her in a matter of fact/concern for your child manner.
If she is just side stepping copays that stinks as she should feel damn lucky to have insurance cover the rest! Some kids don't even have that!
You cannot tell her how to parent either, she is awfully head strong it sounds and even with him not talking it is truly her call. Unless you feel he is in danger or she is holding him back from testing or progress then there isn't much you can do or say really. If you come across articles maybe print them out for her too, that way it isn't your opinion either but something you read! Hugs, hang in there....