I have to say, it doesn't sound to me like your brothers-in-law are taking liberties. Their behaviour seems quite consistent with that which you would expect from your brothers. Showing familial affection is not wrong, it helps to bind people together as a family. If any of them started to make unmistakable sexual advances towards you, you would need to nip that in the bad immediately; unless you actually wanted it to happen. You have given no indication that you are interested in your brothers-in-law from a sexual point view, so you have nothing to feel guilty about, and from the sound of it needed today.
I think your friend may be looking for mischief where none exists. Perhaps she comes from a family where such things were taking place, or perhaps they still are. Her own experiences are just that, her experiences. If she is genuinely concerned, you can tell her that you have made a note of it, and if anything deliberate occurs, you will put a stop to it straightaway. It may also be the case that she is jealous, and jealousy can make people say or do things that can be regretted at a later date. I don't know how many times an ill spoken word has triggered an avalanche of discontent; potentially causing rifts within an otherwise happy family stop
If anything untoward does happen, make it perfectly clear to the perpetrator that you will not tolerate such behaviour. You don't have to make a big drama outfit unless it continues. It sounds to me like you live in a very happy environment, and you wouldn't want to spoil that. Continue living the way you are, and forget these comments from your friend. As I say, she may have experienced things differently from you, and she may have your best intentions in her heart, but you know better than anyone how your brothers-in-law make you feel, so go with your own feelings, not the comments above us.
I'm sure you will continue to live a very happy life, so best wishes.