Ugh. How much cash do they need to get to Cali if they're just going to be staying with someone anyway? Is it possible for you and other family to get them the money right now?
Honestly I'm worried that they will still be firmly planted on your sofa in 6 months time. Remember, these are the same people who said they would only need to stay with you for 3 months and now they've been with you over a year and are starting their family under your roof. If they're saying 6 months that probably means 5 years. The fact they would propose a wishy washy six month deadline means they have absolutely NO sense of urgency.
You have got to do everything in your power to get your sis out before she has the baby, or it will be extremely hard for you to get her out. Once she has that baby all bets will be off. Trust me. She will probably take over your bedroom too because she'll need some space for the baby.
I am very worried about you and your well being. You are dealing with infertility and an unwanted house guest who has taken over your home. That is just all too much stress for one person to deal with and no doubt it is making your health challenges worse.
Both of those things are incredibly hard to deal with by themselves, but at the same time is unimaginable. And as bad as things are now, they are going to get a whole lot worse if your sister is not gone by the time she has the baby. I just can't imagine...
You need to be focusing ALL of your attention on yourself dealing with the mental, emotional and physical aspects of infertility. Your home should be your safe space where you don't have to deal with other people's baby bumps and ultrasounds and the like.
Valeriachan, you have got to get the strength to kick your sister and her boyfriend out ASAP. Whatever it takes. Your sister and boyfriend are horrible, but you also have to accept responsibility for this situation. Stop making excuses and get them out. You have explained why you have let them stay, but I really don't follow your logic. There is no reason why they couldn't be out of your house today. It wouldn't be pleasant, but once it's done you get your life back.