I'm not a mother-in-law officially, but I may as well be for BF1 and BF2.
BF1 has been an item with easy child for 11 years now, they've been living together now for 5 years? Six years? I'd have to sit down and calculate it. He's been a regular visitor to our house since he started going out with easy child. I get on well with his mother but don't see her very often, maybe once very few years. She's a very strong-willed person - rough diamond. Does not suffer fools gladly. Adores easy child, always has. Only has one child and so has invested everything in him emotionally. I'm glad she approves of easy child.
BF2 now lives with us. I have to remind him to visit his family sometimes, although there is no bad feeling or anything, he's just so bound up in easy child 2/difficult child 2. So to a large extent, I have a live-in sister in law. He helps with things when asked but most of the time stays in his 'room' which is also a large work area. He has hobbies in there which totally occupy him, plus he works long hours. He pays board. I know he and easy child 2/difficult child 2 are wanting to move into a place of their own but I don't think she is mature enough yet to cope with the responsibility. We're not rushing them.
Because we have BF2 in the house, I feel parental maybe more than I otherwise would. He has teeth in very bad condition, he will lose them if he doesn't get them looked after. But instead, he puts up with pain until it's too much then goes to a dentist to get them pulled. Not the way to handle it. He should also take out health insurance for dental, to cover his costs, but keeps putting it off. easy child 2/difficult child 2 has her own cover, but until they formalise things they can't have family cover.
I don't nag. Neither do I get overly sympathetic. If he has a sore throat I will suggest a spoonful of honey, but only he can make the decision.
Something very important - do include the non-descendant in discussions too, about plans, suggestions etc. I have to stop myself using my daughter as the information go-between. A family outing coming up - I talk to ALL of them. If I send an email to easy child, I make it to BF1 as well. Sometimes we will send an email to him alone, such as "What do you think we could get easy child for her birthday?"
The other important thing - DO NOT MAKE A FUSS if they say t hey will be at the other parents' for holidays. You can always make a fuss of them when they come over to your place. Make your presence a breath of fresh air for them, not an ordeal.
This year easy child & BF1 will not be spending Christmas with us - they have been with us every year for the last six years or more. It's only fair for him to want to have Christmas with his family - his mother, his grandfather, his uncle's family. easy child was nervous about telling us, but I grew up in a big family, I saw it happen with my siblings. Things CAN be arranged with enough notice, but sometimes you just have to let go and relax about it. A phone call to wherever they are on the day is lovely, a happy celebration with them when they DO turn up is also great. The actual date does not matter.
I really get the pip with people who insist, "We've ALWAYS had Christmas dinner at lunchtime with the whole family, we're not changing now. So if you aren't going to be there for lunch, you are the one ruining our family tradition and we will all be miserable without you."
It's emotional blackmail. Because the OTHER family could be doing the same thing, and it's not nice to be torn. I HATED having to juggle family responsibilities when we first married, it really took the shine off holiday celebrations for me. Watching my sisters go through it too - it really annoyed me. With our big family it was logical to try to organise well in advance and to try to organise one event, but we were still flexible, to try to accommodate everyone. What cheesed me off were the other families, where there might only be one or two siblings, where they changed their minds at the last minute purely on a whim, and THEN made a fuss because we had ten other people already locked in with their other arrangements all made.
Something to think about for holidays - we switched to telling EVERYONE to go have Christmas lunch wherever they felt they needed to be, get it thoroughly out of the other family's system, then come back to our place for a relaxed, laid-back dinner. Summer - we barbecue around the pool or go to the beach, as a family gathering. None of the pretty table decorations, just a paper plate and laminex-topped table. NO fancy shoes, just bare feet.
In winter - we'd kick off the shoes, snuggle up and watch old family movies or slides, or just have a raucous singalong around the kitchen sink.
So keep it relaxed, make you and your place a refuge, love your family and don't impose.
Marg