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Where an individual has died without making proper provision in their Will for their relatives or dependants, judges have a wide discretion to redistribute assets to produce a fair result. There is no magic formula involved although certain factors must be taken into account (eg length of marriage/the assets held by either party). It must be remembered that each case is very different and will be judged on its own individual facts. I cannot of course predict exactly what would be awarded but
where the applicant is a spouse, or a civil partner, of the deceased, ‘reasonable financial provision’ means such provision as would be reasonable in all the circumstances of the case for a husband or wife or a civil partner to receive, whether or not that provision is required for his or her maintenance. The court must consider, but is not bound to follow, the likely settlement that would have made within divorce proceedings, if the parties had divorced rather than the deceased having died.
The daft thing is that the more dependent a spouse is on the other spouse, the more that party will be entitled to claim, on the basis that during the other spouse's lifetime, she was enjoying "being looked after". I note your fiancee does not appear to have any capital/savings, and if you will be maintaining her as well as providing a roof over her head, you will need to make sure that reasonable provision is left for her in your Will. Although a Wills Solicitor can discuss matters in more detail with you, the very least you will probably have to provide to her is somewhere to live for the rest of her life- you are able to grant her such a right in your Will, with the proviso that upon her death, the house is sold and the proceeds split between your 3 children.
I can't really comment on how much capital you woul dneed to leave to her- this would entirely depend on what monies if any she had at the date of your death, and how much you are providing for her during your lifetime.
I would strongly suggest that you speak to a Wills Solicitor, because if you marry, and do not make a Will, your spouse may likely inherit everything and you need to prepare a Will which is fair and reasonable for her but also protects your children.
I hope this assists and sets out the legal position.