My sister makes comments to my kids, undermining me, or questioning my abilities as a parent....?

ok I can tell you this, my mother in law and I had a huge argument out in the middle of the driveway after 10 years of getting along great , we tried communicating our issues by email and my point of view was horribly misinterpreted, she acted like I was attacking her via email calling her a liar etc; this went on for several months, I have found that it is best to speak face to face or by phone on speaker phone if possible so that you can have someone else with you to make sure there are no misunderstandings, get it out in the open and tell your sis that you just have different parenting styles and there is nothing wrong with that