Thank you very much Tiocfaidh, that's very helpful, and has made it all much clearer.

The question wasn't posed because I was worried my husband may try to revoke any of my portion of what he may leave, however.
Our worry was what would happen should (as would be expected) his parents die before he does. We were afraid that I would then have to share our house, business and money with his brothers and sisters. We had hoped that it would be possible for him to make a will such that this would not happen.

From what you say, however, it would appear that in the event of his death, I would be left in a position such that his siblings would be legally entitled to take half of everything we own.
This was what we hoped to avoid.

Could they actually for instance, force a sale of our house in order to claim their share, thereby leaving me on the street?