Tenants in common or joint ownership?

Pizzle - whilst bearing in mind all that Germanshepherdsmum says, I think it might be worth you looking at the following:

www.gov.uk/guidance/hm-land-registry-registration-services-fees

I have recently done 2 first registrations on my own, without a solicitor's help. It is neither expensive nor difficult, you just have to take your time. The Registry are very helpful to members of the public.
That would mean that the 2 houses are at least registered as belonging to you & your sister. That is your first hurdle.

I think that means that is quite easy for you to leave your share to your partner. If not completely satisfactory, it sounds better than the current situation.

I don't think I have overstepped any boundaries here, and wish you luck.

PS: one of the properties I registered had deeds dating back a long time, I just took my time in assembling everything.

Thank you Nannarose, I will certainly have a look at this.

Pizzle, I will send you a pm.