Six weeks will seem a long time at the moment but in the long term it is nothing. Unless she returns again soon, your baby will have forgotten MIL by the time he sees her again. He is too young to know what she is saying. However he will pick up on atmosphere so it is important that you do not get nervous or angry, either in the presence of MIL or if/when talking about her with your husband. Try to tell yourself she cannot harm you or change who you are unless you let her. If you are agitated your baby will be more restless and cry more, and your ability to breastfeed may also be affected. If this happens it can create a vicous circle. If you feel yourself getting worked up, you could try taking some deep breaths and make an effort to relax. You don’t have to agree with what she says, neither do you have to contradict her - just smile and say nothing. If you find she is getting to you, leave the room for a short while ( pretend the baby needs changing, make a cup of tea or just go out in the garden and breathe deeply) and always feed the baby on your own away from her somewhere where you can relax. This is all easier said than done, but remember, it is sad for MIL to only see her grandson for a short time, she will want some happy memories. For you, whatever happens, the time will pass and at the end of it she will be gone again. If you can just learn to trust your own judgement, smile and be friendly, you will all get through it. As has been said, you can listen to her suggestions and advice, say thankyou and ignore them - this is what most men do all the time!
You can be nice and still be your own person, remain detached from her inside, that way there will be no hurt involved.
A great idea to ask about how she parented her son.
Good luck, I hope it will all go well. I am sure you can be strong and get through this. She may be feeling as nervous about it as you are.