No! No! No! YANBU. A cautionary tale:
I was 42, working full time + unsocial hours + main Key Holder. DH, an only son, self employed = no work, no pay. DD1 at Uni, recurring glandular fever, had to be fetched home very ill several times. DD2 at college, far away + homesick. FIL being assessed for Altzeimers, MIL can't cope, collapses, can't get out of bed, can't walk, etc. FIL to respite care, later admitted to care home, MIL won't visit him, too upsetting,arranged home help for MIL but kept getting calls from Home Help or MIL for urgent assistance. EG one at 7.30am MIL collapsed on floor, can't move, has been lying there all night. Mad dash to her house and she is lying there in obvious discomfort, send for Doctor, go downstairs to make tea and discover kettle still warm, cup in sink -WTF!.
A year passes, MIL breaks Hip, hospitalised, will not do exercises, confined to armchair thereafter, commode installed downstairs. My DM has 1st heart attack, Maiden Aunt in law breaks hip. Now 4 rellies in 4 hospitals then 3 home needing cared for. My Maiden Aunt with diabetes develops dementia and at least is admitted to same hospital as FIL which simplifies visiting somewhat.
The pattern continued for 9 years with variations of who needed most looking after. MIL (who hated me, was most demanding, best not to tell her if someone else was poorly as she always upped her care needs. For us, no holidays, no family christmases, no chance of visiting daughters by then living in England. Plenty shopping, visiting, cleaning, washing, accompanying to hospitals, meeting doctors, social workers, and on and on and on....
All five wrinkles died within a 9 month period, leaving only my DF. 3 years later when he died and I was made redundant, aged 54 and menopausal, I had a total breakdown. I feel I was robbed of precious years of prime life time. It was like one of these stage acts where they spin plates on sticks and have to keep them all moving at once.
There is no way on earth that I worked through those years to fund my DD's education only to have them ruin their careers and family life by falling into the same trap. It will be a care home or blue pill for me when I cease to be useful or at least self suffcient.
OP - make a stand NOW or you will be on a slippery slope. Each extra request for help will seem small but it all snowballs and you cannot refuse to help your own parents when you have done so much for the In Laws. Sorry about the long story but I get so angry when people e.g. some posters here, tell you where your duty lies when they have no experience of what that path entails. 'You will be old yourself someday!' Well I am old now and I say NO! I would not allow another woman to make that sacrifice for me.