How to send a message to family of they don't mind to be sponsors at my wedding?

What do you mean by "sponsors"?

In this day and age it is your responsibility as a couple to pay for a wedding within your own means. Unless your parents or your in laws were to offer to contribute, host or co-host, it would be rude to ask. The most I'd feel comfortable with would depend on what has already been discussed and offered on their parts, either now or in the past, not solicited by you. In that case you can ask them to let you know how they would like to be involved. That doesn't necessarily imply paying.

Even if they have made generous, unsolicited offers in the past, a message is not an appropriate way to discuss. Pick up the phone or visit in person to discuss if they are local.

By "sponsor", do you mean like a "padrino"? Hopefully someone here can help if that's your question.

Also wondering if you meant this (or ninang/ninong in Filipino culture). Is there any reason in particular that you feel like you need to ask again? I'm just curious because my sponsors were super excited when I asked them, so I never had a reason to think I needed to ask again.

From what I know of sponsors, they are not financial but couples with whom the newleyweds can go to for advice or counsel for the duration of the marriage. If this is what you mean, I would call them since you are close. Later, find a thank- you card with an already- written touching message that fits the significance. Best wishes.

The financial obligation is often a significant part of it. Thank you notes should be written by you, in your own words, not Hallmark's.

Knowledge of western or American etiquette doesn't cross-over to cultural expertise.

There's always some nosy busybody inquiring about sponsors at Filipino weddings, often looking to feel superior themselves. But, I have never taken part of or been a guest at a wedding where spiritual elders contributed financially. Hand outs would be rather contrary to familial pride. Can't speak for padrinos and South American cultures myself. Of course, the OP never replied as to the type of sponsors involved.

How would you know as a guest what anyone contributes? I'm familiar enough with the custom to know that financial sponsorship is often part of the deal. OP can explain if she's not requesting money and I'd happy for the assumption to be wrong. But why else would she assume they had forgotten after two short months if it had nothing to do with financial support?

If this is long standing tradition in both families then I imagine the "ask" is mostly ceremonial, which did not entirely seem to be the case since OP sounded nervous about how to word a second request/reminder. Unless OP explained the cultural aspect it's likely she will get advice that reflects western etiquette when posting on an etiquette board that was started here and leans very heavily to that demographic. Namely, that it's rude to ask anyone who hasn't offered on their own to contribute or pay for your wedding.