They cannot stop you from speaking your native language, nor stop your mother from speaking it, nor stop either of you from speaking it to the children. But in any family, people need to be respectful/polite, trust each other, and compromise and that's apparently not happening here.
If you're fostering the children being bilingual, then logically it makes sense to extend the "rules" about languages to their grandparents. They speak your language with you, your mother, and whatever other relatives on your side or friends or community speak that language and choose to use it. They speak English with their dad, paternal grandparents, guests in the paternal grandparents' house, etc. Since you're living in a country where English is dominant, there will of course be times when everyone is speaking English out of courtesy or convenience. If your mother doesn't speak English well enough to keep up, then they have to accept that there will be asides in your language between the two of you and perhaps between her and the children.
Have you explained to your husband and his parents what the specific issue was that caused you to have that conversation with your mother in your language in front of them? They need to address/refer to your mother by her name as she has repeatedly asked, and assuming they have reasonable powers of understanding, they should be able to grasp that their behaviour was so odd that your mother reverted to her native language to ask for your help, as she had tried repeatedly but was not being understood by them when she spoke English and was distressed about this.
Your husband needs to be more involved and support you here: for the 30 minutes a day and the Sunday lunches when everyone is together, the main language will be English but there may be snippets of the other language if needed, which you or someone else who speaks both languages fluently can then translate if that makes them more comfortable. If the in-laws make this impossible, then perhaps it does make sense to cut down on the time you and the children spend with them, and/or see them and your mother separately.