Could you provide your own seat for your child to use in her car? And explain your child is too small for the booster and you would prefer they use their own seat?
I agree with other comments, you can talk to her about concerns facing your LO but I wouldn't mention anything about her children as that's her business and it doesn't sound like your close enough to express concern without causing offense.
@Bethany her car is too small for our seat 😕
Is she not willing to take out the high back booster to put your kids seat in? Your child's safety in the car is more important than anything else. I'd just say that that seat won't work for him cause he's too young. In some states it's not even legal for a 2 year old to be in a high back booster. Is it possible she just doesn't know much about car seats.
I am sorry, but my kids' safety comes first, no matter the circumstances. I would definitely tell her it's not safe for your kid to be in a booster seat, either take your own car seat or walk. And its not safe for under 4 to be front facing purely because with even a small break (no accidents) it can break their neck.
I personally think that if you were to talk to her about any of it, only talk to her about your concerns regarding your kid. Don't bring up your issue with her kids because that's really not your business or place to do so. It's very valid for you to tell her your child isn't old enough/doesn't weigh enough for the booster and don't feel comfortable with your child not in the car seat you feel comfortable with.