I'm sorry you got a stern reply from someone when this was your first post on this site. Every now and then there's someone who jumps in too quickly for whatever reason.
You don't post the ages of your children, and I think it's interesting that your SIL seems to be getting into one relationship (the one with your daughter) even more than the other kid(s). Not sure if that's because of the age of your daughter or your SIL's perceptions/advice about girls in particular. I wonder if she has a strong need in that area - in which case, sometimes pitying her can make you feel a little stronger and more secure.
I agree with the others who have said it's wise to try to salvage & value family members' relationships with your kids, while still asserting your right to make the decisions. If your kids are old enough to see your SIL undermine you by offering advice that is contradictory to your views and values, then you have to decide whether to step in right then, in front of the kids, or defer it until you can take your SIL aside.
You can start out gently and with a little humor, saying something like "Susie really needs an auntie. She already has a mommy." If that doesn't work, you can say "Thanks. I'll consider your advice." If she continues, just repeat it, "Like I said, I'll think about it." Some people, rather than say anything, like the "silent stare" approach - just look at the advice-giver with a quizzical stare and say nothing (not baffled or confused, because you need to look confident, more with a facial expression of "I can't believe you said that!") and then follow with a bemused smile or small chuckle.
You can also be extremely direct and firm, if you feel comfortable doing so, but that may risk alienating her. And you have to be strong enough to stick to your position and to withstand any backlash. That doesn't mean it won't work, but you have to be very clear about it up front.
Also, a little tip - if you can put a little more info in your post, that helps. If you want to add more info afterwards, you can click "So What Happened" which lets us all see very clearly what you've added because Mamapedia posts it right below your original question. If you post an answer (as you did below), it gets into the overall mix and others might not read as carefully and realize that it's actually from you rather than another "random" responder. If you post a question that gets 15 or 20 or 30 responses, new responders don't always read them all.