I am trying to keep this as short as possible and to avoid turning this post into a rant about SIL, who is actually a good person in general, with many admirable traits. The problem is that SIL moved across country several years ago, still keeps in contact with her old friends here in HomeState, usually visits during holidays/special events, and typically has a very full social schedule when she's in town. She has a Good Friend who SIL continues to invite to family gatherings. The problem I have with this is that I am the one hosting, cooking, and paying for these gatherings. I find it irritating to have a random woman sitting at my table eating Christmas Dinner (even if it's served closer to New Year's, that's when we are celebrating the holiday as an extended family), not to mention the rudeness of SIL either ignoring the family to chat with her friend, or ignoring her guest who knows no one else at the house to talk to family members. But I have never been able to bring myself to turn someone away from my door at the holidays. I've also had problems addressing it with SIL, preferring to hope that she won't do it again.
Last summer, DH graduated from college, and I held a party that ended up being at a park. Good Friend turned up, and I literally didn't recognize her when I saw her in the street.
Christmas of 2010 I tried a very passive approach to talking to SIL. I mentioned that I ended up eating a cold dinner because I was scrambling to throw together extra side dishes because of the unexpected guests. That's when I found out that Good Friend lives only a few miles from my house and SIL hadn't been able to arrange any other time to get together with her.
After the graduation party, SIL was complaining that Good Friend hadn't even spent much time with her. GF's son kept running off to other parts of the park and kept trying to join another birthday party. GF was constantly either running after him, or taking care of her baby. I told SIL that I didn't know why GF was even there. SIL made some noises about not thinking I would mind. My tone got a bit cold and I told her I did. Later MIL was telling me how sad and depressed SIL was that Good Friend had ignored her, and I said I didn't really want to hear about it. MIL scolded a bit (not in a mean, bad way, it's just that sometimes she forgets that I'm not her actual child, and that I'm not a teenager for her to correct

though she did a lot more raising of me back in my teen years than my mother did) and told me that SIL only has a limited amount of time in Home State, and it's hard for her to make time for everyone. My response was that SIL could choose to spend her time any way she wanted, but if hanging out with her friend was more important than DH's celebration, I wished they would just go to lunch and save me the hassle of feeding everyone. It's like neither of them got that having Good Friend at the party, and socializing with her there meant that SIL was NOT socializing or celebrating with DH. Nor was she socializing with the rest of the extended family: aunts/uncles, cousins, grandparents, kiddos.
The whole family is coming out for Easter weekend. BIL #1 has been looking for a job in HomeState, and has been working at an internship for the past month. He will still be out here for Easter, and their kids have Good Friday off from school, so SIL decided that was a good time to visit. MIL had been planning a trip for Feb, but decided to delay and travel with SIL + kids. FIL couldn't have come in Feb, but work will be slowing down, so he will be able to make it out for Easter. Taking all the scheduling into consideration, DH and I decided to postpone having our Baby Blessed in our church so that MIL/FIL could be here.
So, I will be hosting Easter dinner, and it will be a combination Easter Dinner/Blessing luncheon. There will be lots of family, and I'm worried that SIL will invite Good Friend again. Should I preemptively assume that SIL will invite GF and tell her before they even get on the plane that GF is not invited? I don't see how to work that into a conversation, and it seems off (or maybe just terrifying) to call SIL up out of the blue specifically to tell her that she is NOT to invite GF to my house.
If SIL does invite GF anyway, how do I turn her away? I can't imagine simply shutting the door in the face of someone who was improperly invited. It seems like punishing the innocent bystander. After all, she doesn't know that her presence is irritating me. For all I know, she's asked SIL if I'm ok with her being there, and SIL has assured her that it is fine. Should I just think of her as collateral damage to SIL's actions?