You need to think very carefully together what you and your partner want to do in the future. As Clubman says the value of your home and mortgage status and potential inheritance of your partner are irrelevant.
You sound like you are both in a committed relationship and see yourselves together for life. So the simplest way forward is for you both to marry and you make wills leaving each other any assets on death. You and your spouse can also stipulate that on death, if the other spouse has predeceased then all goes to the 5 nieces and nephews. Job done, nothing to worry about.
If you do not marry and if you make a will leaving your partner a right of residence but leaving the property to the 5 nieces and nephews. You die at 50 and your partner lives another 50 years and dies at 102. What happens during those 50 years? Will the 5 maintain the house, pay the insurance, upkeep and taxes. After all they are the owners. Or will the oldest at 30 say, I am getting married, I want my cut now, and try to sell the house, make it difficult for your partner to continue to live there? Or will the youngest now aged 7 realise at 25 they have to pay local property tax, which has skyrocketed for houses not being lived in by the owner and the roof is leaking and needs major repairs and this is all because of an aunt they barely remember who wants this 70 year old to live in their house until he dies. All of these things can and do happen. It is impossible to predict the future.
Or what if you are 80, and your partner 82 and you end you in a nursing home for 3 years and your estate needs to pay 21% of the value of your assets for fair deal within 12 months. And the only way it can happen is if the house is sold so your partner is offered 30% of the value of the house as a fair value for his right of residence and he leaves, the house is sold, and the 5 nieces and nephews get 10% of the value of the house each, revenue get 20% and your partner gets 30%, but is 85 and has no home to live in.
So in essence you need to get advice from your solicitor on what is the best approach to take that gives you what you want, a home for your partner to live in, in all circumstances.
If you just say to your solicitor, I have a house I want to leave to my nieces and nephews when I die and I want to give this stranger (not a relative) the right to live there until he dies, then that is what you will get but it may not serve any of you as you wished dependant on future circumstances, which you cannot predict.