Dina,
You've thought out all the current and possible future issues, which is a good start. Welcome to the roller-coaster world of international romance!
To address your concerns:
1. Obviously, no matter who moves to which country, one of you is going to be away from your family for prolonged periods. You and he should honestly (and separately) make a list of the pros and cons of each of you moving to the other country. (To give you some background: I'm American and came to England five years ago to meet a man whom I'd never seen face-to-face. We met over the net, started sending each other several emails every day, exchanged pictures, started "chatting" on Yahoo Messenger, then started talking on the phone for hours every night, before we decided it was silly and we should simply meet and find out whether we clicked. Bear with me--I'm not making this about me--I'm just telling you my own experience. I was supposed to visit for just two weeks, and ended up staying for five months, at which point we got married at a registrar's here. I'd had all my belongings put in storage. I was fortunate enough for employment not to be an issue--I'm self-employed and can work from anywhere, as long as I have a computer and internet connection. If we'd given it more thought, we'd have married in the States, as I'm from a large family (both immediate and extended), whereas he, his parents, and one cousin are his only family in England. Also, his parents are 60 and 61, whereas mine are 79 and 77--it would have made more sense to be in the States. Then again, obtaining a visa in the States is MUCH more difficult than it is to do here.) Now, back to your situation: Don't jump the gun by already thinking about what will happen if you have kids in the States, while your mother and sisters are still in the UK. One step at a time...
2. You didn't say what you do. Is it anything you could do on your own/without having a "9 to 5" employer? Even if not, most employers would understand a gap if you'd moved from one country to another.
3. Your boyfriend's concerns are valid, as far as his mother's health goes. I speak from experience, as my mom has, in the five years I've been here, gone from being seemingly normal to rapidly losing her memory--she's in the midstages of Alzheimer's. Even though four of my five brothers live in the D.C. area (and the fifth will be moving back in a couple of months), I suspect that between them and my dad, they'll put her in a nursing home if it becomes far too difficult to care for her. Since she's repeatedly requested a promise from me to never let her be put in a nursing home, no matter what (a promise I've repeatedly made), I understand your boyfriend's predicament of what might happen with his brothers if he's not there to intervene.
4. As far as your possible wedding goes (and I do have a bias, as I never had the fairytale fantasies, nor did I give a sh-t about the pomp and circumstance of the Charles and Diana wedding), should you decide to go through with it, keep it simple and go to a registrar's. Down the road, you can do something more formal (combined with a renewal of vows, perhaps)--that's our plan, anyway. If go to a registrar's initially, you'll be spared all the stress of arranging for all the details involved in an out-and-out wedding. The point of a wedding is to formally establish your commitment to one another--not to put on a big show to please the parents and put yourself in debt. Do that when it's more practical.
5. I think the most important thing, though, before you even consider any of the above, is for you to spend more than a couple of weeks together. It's very easy to romanticize/idealize a relationship when you're together for short periods of time. In the early stages, both people are on their best behavior, you really don't get a chance to see what the person is like day to day, the "parting is such sweet sorrow" rule applies, etc. When you spend a considerable amount of time with someone--enough to get a realistic view of what he/she will be like to live with, day in and day out, and after the initial electric romantic feelings fade into loving, committed friendship--then you've allowed yourself to decide whether you LIKE as well as LOVE him/her, and whether you can truly see spending the rest of your life with him/her. Again, everything about someone you fall madly in love with seems endearing in the early days--you need to take a long view not only about your work and your families, but also about the blunt realities of day-in, day-out life together.
6. DON'T PANIC, to quote the old T-shirt. Think things through, both with him and on your own.
Suzanne
P.S. Believe it or not, I'm a romantic idealist at heart. I just hate to see you so overwhelmed, to use your own word, when it's early days. Feel free to send me a PM.