I’d say that having her round at the weekend once a fortnight isn’t a big thing. It’s been less than a year and she’s lonely.
However, you can choose the meal and if you want it to be low-key and doing other things rather than sitting in a circle chatting all afternoon, that’s your choice. It’s up to you to set the tone of the afternoons.
If a roast is too expensive, tell your DH to tell her next time he invites her ‘Woukd you like to come over for lunch on Sunday. We will be having pasta this week’ if it would help to prepare her in advance. Or it might be best not to warn in advance and that’s just what you dish up.
The food is totally up to you. Even if she comments on it not being a roast, you can just breezily say ‘oh we don’t tend to eat roasts so much now’ or ‘we like to mix it up a bit at the weekend’ or if you want ‘we don’t have roast every week as everything’s gone up in price so much. We have it sometimes but not as regularly as we used to’.
In the afternoon, it doesn’t have to be a case of sitting on ceremony. You can be pottering about as can the rest of the family - in and out. If someone needs to explain that to her, it’s easily done by your DH
’Mum, we’re really glad to have you with us each fortnight. You’re feeling like part of the furniture now, so hope you don’t mind if we do a bit of pottering about and getting some of the bits and bobs we need to do for the week ahead done, rather than sitting all afternoon’.
Make it work for you. She’s lonely and coming round twice a month isn’t a big ask for a widow who’s not even reached the year mark yet. Accept that, but make it work for yourselves too.