Got a problem with a sister in law

So I'll try to explain it as short as I can.  I've been living in the US for 4 years I got married with my husband an he brought me here, I dont have many friends and since I came here I have to rely on my sister in law cause she has been helping me in many waysto understand my way around, sometimes has been good cause she is a girl so we can relate to each other, but the only problem is that she is VERY bossy, she has to get her way, her temper is stroger than her moms, dad's and even my husbands. So the problem has been since I gave birth to my son she helped me a lot cause I wasnt driving at the time, but i noticed she started to make some comments like "why dont you give me your son" or one time when we lost insurance cause my husband stwiched jobs she told me to sign my kid to her and she could give him insurance. One time we were thinking about moving to NC and she told me that it was a shame cause I would have to miss my kid cause he would stay with her. 
Recently she has been not respecting my authority as a mom like if I say no to my kid getting candy before dinner she goes and give it to him, she is constantly telling him to go to her house and when I say no my kid gets really upset and I end up being the bad guy! I feel she is taking over my roll as mom and Im tired, I constantly tell her not to do it and she wont care I just dont know what else I can do!