Hmmm~ I feel that the child's parent is the one in control - not the other family members. For us, we set up the rules and ask that they are followed at all houses to avoid confusion/ frustration on all involved. (Such as my nephew is allowed to jump on furniture at grandma's but not at home - at an early age, I feel they cannot always make that distinction and it is a headache for Mom & Dad to break them of the habit, right?)
It sounds like your mom is experiencing some of her own issues and is taking them out (in a way) on you and your little one.
I agree where this is your turn to raise your baby. My Sister-in-law and a few others started to barrage me with much advice in the beginning when my little guy was born, but I basically tuned it out and made it clear that if I didn't ask you for it - I do not want it. Hubby was a bit better about the whole thing, of course.
I see nothing wrong with your daughter playing with cars or any 'boy' toys. They are just as much fun for girls as they are for boys... Besides - even Barbie has a car! :)
I think you should (if you have not) have a sit down talk with your mom without the baby there as a distraction and tell her that this is going to the extreme and tell her how you feel. If you need to, have your aunt sit with you as she also sees it.
I would hate to turn to using the baby as an ultimatum, but if your sanity and relationship are already strained, then it may have to come to the threat of it to get the point across.
Trust me... you are not the only one with these issues, so I am sure you will get some other advice. Mine is just trying to reach her and tell her that she needs to back off and let you and your husband set the scene and make the call and she is the follower, not the controller.
Breathe, stay calm and good luck!