Lina should give her son and future DIL books on boundaries, as suggested.
At work, Lina should tell Sylvia that she doesn't want to talk about the wedding on company time. (And she should not talk about the wedding with any other co-workers either, so that they don't get dragged into this either.)
Anytime Anna asks for advice, Lina's response should be "What do YOU want to happen?" and based on her answer, help her find a solution to make what SHE wants happen. I can understand how the long term relationship they have makes her to the go-to person for Anna to seek advice. I have a similar relationship with my mom's BFF's and have often consulted them or vented to them because they understand her like I do. But I never asked them advice and they never offered it. We did more commisserating than exchanging. I think that's what Lina should do. Be a sounding board, but don't try to fix this. Only Anna can fix this.
The NYE story angers me, but we've seen many posts on Ehell about uninvited guests showing up and posters needing help polishing their spine to turn people like this away. I don't think this is immaturity, it's just lack of knowledge and spine on how to handle situations like this. I'd help them polish their spine with tips like "You don't HAVE to answer the phone every time it rings. No is a complete sentence. It's okay to say no, this doesn't work for me. And teach them how to suggest alternatives or figure out a compromises. If the parents drive 5 hours for a surprise visit, compromise and say "Gee, you should have called. We're on our way out to a party. You can stay here and watch TV until we come back, or rest before heading back if you can't wait for our return." Better yet, Anna should call her mom now and say " You know that surprise visit on NYE? I didn't know how to handle the surprise then, but now that things have settled I have to tell you that we missed a party because of that because we didn't know how to tell you we needed to leave and we stayed to visit with you instead. It upset our friends, who expected us, and it upset us. I should have said something then, but I didn't know what to do. I've thought about it so I'm calling you now to say, Please, in the future, do not do that again. If you are going to visit, please arrange it beforehand. If you are in the neighborhood and decide to pop in, call and check beforehand to see if I'm free. I will not be rude and end as a no-show for something I committed to simply because you made a surprise visit."