Your brother is likely near your age; since he should expect to work into his mid 60s, do you really think that he wants to support someone who retires so relatively young. Additional, once he is retired, his income will be greatly reduced, and he will need to support his immediate family as well as you two. I wonder if he considered his financial responsibilities when he sponsored you; most likely not.
You need to do the numbers. Look at the US poverty income numbers for 2 people in your area. Double it to get a moderately comfortable standard of living (or at least 1.5). Assume a 3 percent rate of inflation (potentially very low) and calculate the cost of living to age 90 (30 years in your case). Take your assets you are bringing to the US; apply a 6% rate of return (currently high with the current interest rates; 3-4% would be conservative at this time).
Alternatively calculate expected expenses. Don't forget to include medical insurance and copays and prescriptions at full cost. If you have preexisting conditions requiring non-generic maintenance meds (a common drug for type 2 diabetes is nearly $300/mo and that is cheap compared to memory drugs, etc.), these can very easily be more costly than the insurance premiums; I would plan for total medical costs for 2 people in early 60s to be $1500 to $2000 per month. You can find many budget worksheets so you know what to include. As young as you are, you will need to be generous with your travel budget. Another alternative is to ask your brother what he planned as his retirement baseline income number and use it.
Check the numbers. Can you support yourself? If not, what is the shortfall? Can your brother honestly support you and his own family?