This. Sounds like he's never cut the umbilical cord. When he got together with you, you and your DC became his primary family, the one he was born into should have moved into second place. Instead your lives are revolving around his mum!
If he has actually gone as far as to call you selfish for not wanting a stairlift, then you definitely have a DH problem and I'd have gone ballistic at my DP for that comment. It's so very unjust and manipulative.
You are not selfish for wanting your home to primarily work well for those who live there.
I wouldn't be entertaining a commode either! By the time someone reaches that level of disability, they've gone beyond the stage where overnight visits to other people's houses are practical.
If she actually needs DH or someone to stay overnight with her, then she's at the point where a care home is more appropriate. Unless she has the funds to pay for an overnight carer.
If she doesn't need that level of care, why is he staying over every week? It's way beyond excessive. Like he's never properly left home! Is she controling and he's under her thumb? You're married adults, not teens and one of you rents a bedsit while the other lives with parents. You shouldn't be sharing your DH like this.
Sorry OP but she's moving in by stealth. Commodes and stairlifts and weekly visits! Once every two months maximum is reasonable - and she comes with nothing other than her walking aids and overnight bag. You need to crack down on it. No caring-related stuff left in your home and no weekly visiting. She shouldn't be dumping her morning wash routine on your DH (or you? Sorry NRFT) when she's paying carers to come to her home to do it, she should be in her home ready to receive the care. You deserve to come first (after the DC needs) to your DH. The easier you make it for her to stay over the more she'll do it and the more your lives will revolve around her. You and DH will end up prioritising her instead of each other (he's already doing it!) and your teenage DC won't get much of a look in. It's damaging to DC and to your marriage.