Answer: As tenants in common, though you own the property jointly, you each own a specific share: in your case, 50 per cent. On your, or your partner's death, the deceased's half won't automatically pass to the survivor, but will pass according to the deceased's will or the rules of intestacy.
Your partner and his family will have to decide whether the contribution they made was a gift or a loan but it won't affect your ownership of the property. Nevertheless, if the family wants to be repaid and your partner can only afford to repay them by selling his share of the house — and you don't want to sell yours — then there could be an issue.
Establish if your partner can fully explain the nature of the arrangement he has with his family and if there is a declaration of trust covering their contribution.
Consider entering into either a cohabitation agreement or a declaration of trust of your own with your partner. This could deal with an eventuality such as only one of you wishing to sell and cover how the property should be valued.
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These answers can only be a very brief commentary on the issues raised and should not be relied on as legal advice. No liability is accepted for such reliance. If you have similar issues, you should obtain advice from a solicitor.