My sister wants MY children

So my older sister & her husband are unable to have biological children. They plan to adopt but the cost is holding them back right now. I suffer from unexplained infertility. I have 3 children (it just takes me about 4 years to get pregnant with each one). I'm now pregnant with #4. I go out of my way to downplay my pregnancies so I don't upset my sister. So, every time I have a baby she says "can I have it? You already have children. I'll take this one"!
She seems to be completely serious! I don't know how to respond & just end up giggling nervously & going to another room. Also whenever she sees my children getting rowdy (which is often b/c they're boys & there are 3 of them) she says
"I could adopt him. Really, we can just sign the papers".
My children aren't dogs from the spca. We love them all VERY much & all my pregnancies have been planned & wanted. I'm 35 years old, financially secure & have been married for 13 years. I really try to avoid her because I feel like she must think I'm a bad mother or something & that she could do better with my children. I don't know what to say to her to make her stop asking for my kids.