You are obviously free to leave gifts in your Will to people who are not in your immediate family (i.e. other than your partner or children). And the only restriction on this is that you cannot normally disinherit your close family and dependants – you must make sure that they are adequately provided for.
As with gifts to children, it is possible to give gifts to other family members either by description, or by specifically naming the person who you would like to benefit. As long as you are sufficiently clear, you can describe who you would like to benefit however you wish. However, the courts tend to interpret some phrases in particular ways.
‘To the family of’ a person: this is generally interpreted as a gift to the children of that person. If you intend the gift to go to a wider group, it is important to make this clear by adding additional information.
‘To my relations’: this is presumed to refer to the people entitled to inherit under intestacy, apart from your spouse or civil partner.
‘To my nearest relations’: this is interpreted in a similar way to ‘to my relations’, but it only includes people who are blood relations.
With all of these phrases, it is possible to give them a different meaning by fully explaining your intentions in your Will.
As you can see from this, the usual meanings of these phrases do not include in-laws. For example:
‘My brothers’ only includes your own brothers, not your spouse’s or civil partner’s.
‘My grandchildren’ does not usually include your spouse’s grandchildren through a previous relationship. That may be different if you have become accustomed to referring to them as your grandchildren.
‘My nieces and nephews’ means the children of your own siblings, not your other half’s. However, if you generally refer to your partner’s nieces and nephews in that way, or if you don’t have of your own, your partner's nieces and nephews may be taken to be the intended beneficiaries.
If you want your in-laws to have a gift of a share in your estate, you need to be explicit. ‘My civil partner’s brothers’, ‘my sister’s husband’ or ‘my wife’s nephews’ would be clear.