If you want to be proactive, have a family meeting with your siblings and start a conversation about YOUR parents. Be prepared, some people will have money to give, some will have time and some will have neither. It's hard to predict the future, but your first responsibility is to your immediate family ( husband + kids).

You need to find them an affordable place to live. Low income senior housing.

This idea that a normal middle class couple can afford to save for their own retirements, raise their kids, keep a roof over their own heads and then turn around and support their parents is crazy. Even if we wanted to do that it just is not possible.

We aren't exactly spring chickens ourselves, our home is in a remote area so moving an elderly person in with us is just not going to happen - ever. And we have always (for decades) been upfront about that. We will visit them, we'll keep in touch and we love them but they need their own place on their own budget.

There is no easy answer. If you have a good relationship with your SIL everyone should contribute what they feel they can afford to. There is no formula.

I have 3 young kids, to add

Both of my parents & ILs are retired (do not work), and they are all over 65 years old. ILs have decent social security benefit while my parents have much less social security benefit. They all do not have any debts (mortgage/credit card/car payment). DH has 1 sibling (married with 1 young kid), and SIL is the only child to her retired parents. I have 2 younger siblings (single as of now without kid). Everyone works (except parents), and everyone live local within 30 mins driving distance. SIL (only child) seems have no choice but to take care of her own parents if they are sick one day. If one of both IL health fail, how/who to share the burden in expense sharing & responsibility (take care) sharing? And, if one of both of my parents health fail, how/who to share the burden in expense sharing & responsibility (take care) sharing)?